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* Thin, well-typed wrapper around the MailChannels Email API.
*
* @see https://docs.mailchannels.net/email-api/sending-email/email-intro
*/
import { MailChannelsError } from "./errors.js";
import {
Attachment,
ContentBlock,
DkimConfig,
EmailAddress,
MailChannelsClientOptions,
Personalization,
SendEmailOptions,
SendEmailRequest,
SendEmailResponse,
} from "./types.js";
const DEFAULT_BASE_URL = "https://api.mailchannels.net/tx/v1";
const SEND_PATH = "send";
const REQUEST_ID_HEADER_CANDIDATES = [
"x-request-id",
"x-mc-request-id",
"mc-request-id",
"cf-ray",
];
/**
* Minimal subset of DKIM properties used for validation when normalising payloads.
*/
type DkimFields = {
readonly dkim_domain?: string;
readonly dkim_selector?: string;
readonly dkim_private_key?: string;
};
/**
* Checks whether the provided value is a non-empty string after trimming whitespace.
*
* @param value - Value to test.
* @returns True when the value is a non-empty string.
*/
function hasText(value: unknown): value is string {
return typeof value === "string" && value.trim().length > 0;
}
/**
* Converts response headers into a lower-cased record for stable lookups and serialization.
*/
function serializeHeaders(headers: Headers): Record<string, string> {
const result: Record<string, string> = {};
headers.forEach((value, key) => {
result[key.toLowerCase()] = value;
});
return result;
}
/**
* Extracts a request identifier from well-known header names used by MailChannels and proxies.
*/
function extractRequestId(headers: Headers): string | undefined {
for (const header of REQUEST_ID_HEADER_CANDIDATES) {
const value = headers.get(header);
if (hasText(value)) {
return value.trim();
}
}
return undefined;
}
/**
* Parses an HTTP `Retry-After` header into seconds when possible.
*/
function parseRetryAfterSeconds(headers: Headers): number | undefined {
const headerValue = headers.get("retry-after");
if (!headerValue) {
return undefined;
}
const numeric = Number.parseInt(headerValue, 10);
if (Number.isFinite(numeric)) {
return Math.max(numeric, 0);
}
const parsedDate = Date.parse(headerValue);
if (Number.isFinite(parsedDate)) {
const deltaMs = parsedDate - Date.now();
if (deltaMs > 0) {
return Math.ceil(deltaMs / 1000);
}
return 0;
}
return undefined;
}
/**
* Derives a user-facing error message using JSON payload hints or textual fallbacks.
*/
function deriveErrorMessage(
status: number,
statusText: string | undefined,
parsedBody: unknown,
rawText: string
): string {
const fallback = statusText
? `MailChannels request failed with ${status} ${statusText}`
: `MailChannels request failed with status ${status}`;
if (parsedBody && typeof parsedBody === "object") {
const message = extractMessageFromBody(
parsedBody as Record<string, unknown>
);
if (message) {
return message;
}
}
if (hasText(rawText)) {
return rawText.trim();
}
return fallback;
}
/**
* Extracts a meaningful message from common MailChannels error payload shapes.
*/
function extractMessageFromBody(
body: Record<string, unknown>
): string | undefined {
const candidateKeys = ["message", "error_description", "detail", "error"];
for (const key of candidateKeys) {
const value = body[key];
if (typeof value === "string" && value.trim().length > 0) {
return value.trim();
}
}
const nestedError = body.error;
if (nestedError && typeof nestedError === "object") {
const nestedMessage = extractMessageFromBody(
nestedError as Record<string, unknown>
);
if (nestedMessage) {
return nestedMessage;
}
}
const errorsField = body.errors;
if (Array.isArray(errorsField)) {
for (const entry of errorsField) {
if (typeof entry === "string" && entry.trim().length > 0) {
return entry.trim();
}
if (entry && typeof entry === "object") {
const nested = extractMessageFromBody(entry as Record<string, unknown>);
if (nested) {
return nested;
}
}
}
}
return undefined;
}
/**
* Performs structural validation to catch common mistakes before hitting the API.
*
* @param payload - Candidate payload to validate.
* @throws {TypeError} When any required field is missing or malformed.
*/
function validateSendRequest(payload: SendEmailRequest): void {
if (
!Array.isArray(payload.personalizations) ||
payload.personalizations.length === 0
) {
throw new TypeError(
"`personalizations` must contain at least one recipient block."
);
}
for (const [index, personalization] of payload.personalizations.entries()) {
validatePersonalization(personalization, index);
}
validateEmailAddress(payload.from, "from");
if (payload.reply_to) {
validateEmailAddress(payload.reply_to, "reply_to");
}
if (!Array.isArray(payload.content) || payload.content.length === 0) {
throw new TypeError("`content` must include at least one content block.");
}
for (const [index, block] of payload.content.entries()) {
validateContentBlock(block, index);
}
if (payload.attachments) {
for (const [index, attachment] of payload.attachments.entries()) {
validateAttachment(attachment, index);
}
}
validateDkimFields(payload, "request body");
payload.personalizations.forEach((personalization, personalizationIndex) => {
validateDkimFields(
personalization,
`personalizations[${personalizationIndex}]`
);
});
}
/**
* Ensures each personalization includes valid email addresses.
*
* @param personalization - The personalization object to validate.
* @param index - Index within the personalizations array for error reporting.
* @throws {TypeError} When required recipient fields are missing or malformed.
*/
function validatePersonalization(
personalization: Personalization,
index: number
): void {
if (!Array.isArray(personalization.to) || personalization.to.length === 0) {
throw new TypeError(
`personalizations[${index}].to must contain at least one recipient.`
);
}
personalization.to.forEach((recipient, recipientIndex) => {
validateEmailAddress(
recipient,
`personalizations[${index}].to[${recipientIndex}]`
);
});
personalization.cc?.forEach((recipient, recipientIndex) => {
validateEmailAddress(
recipient,
`personalizations[${index}].cc[${recipientIndex}]`
);
});
personalization.bcc?.forEach((recipient, recipientIndex) => {
validateEmailAddress(
recipient,
`personalizations[${index}].bcc[${recipientIndex}]`
);
});
}
/**
* Ensures an email address object contains a non-empty email field.
*
* @param address - Address to validate.
* @param path - Descriptive path used in error messages.
* @throws {TypeError} When the email field is absent or blank.
*/
function validateEmailAddress(address: EmailAddress, path: string): void {
if (
!address ||
typeof address.email !== "string" ||
address.email.trim().length === 0
) {
throw new TypeError(`${path}.email must be a non-empty string.`);
}
}
/**
* Verifies each content block includes a MIME type and string value.
*
* @param block - Content block to validate.
* @param index - Index within the content array for diagnostics.
* @throws {TypeError} When required fields are missing or invalid.
*/
function validateContentBlock(block: ContentBlock, index: number): void {
if (
!block ||
typeof block.type !== "string" ||
block.type.trim().length === 0
) {
throw new TypeError(`content[${index}].type must be a non-empty string.`);
}
if (typeof block.value !== "string") {
throw new TypeError(`content[${index}].value must be a string.`);
}
}
/**
* Ensures an attachment declaration satisfies MailChannels requirements.
*
* @param attachment - Attachment metadata to validate.
* @param index - Attachment position used in error messages.
* @throws {TypeError} When MIME type, filename, or Base64 payload are missing.
*/
function validateAttachment(attachment: Attachment, index: number): void {
if (!attachment.type || typeof attachment.type !== "string") {
throw new TypeError(
`attachments[${index}].type must be a non-empty string.`
);
}
if (!attachment.filename || typeof attachment.filename !== "string") {
throw new TypeError(
`attachments[${index}].filename must be a non-empty string.`
);
}
if (!attachment.content || typeof attachment.content !== "string") {
throw new TypeError(
`attachments[${index}].content must be a base64 string.`
);
}
}
/**
* Confirms that DKIM properties are present and non-empty.
*
* @param fields - Object containing DKIM values to check.
* @param path - Human-readable location for error reporting.
* @throws {TypeError} When any DKIM field is missing or empty.
*/
function validateDkimFields(fields: DkimFields, path: string): void {
if (!hasText(fields.dkim_domain)) {
throw new TypeError(`${path}.dkim_domain must be a non-empty string.`);
}
if (!hasText(fields.dkim_selector)) {
throw new TypeError(`${path}.dkim_selector must be a non-empty string.`);
}
if (!hasText(fields.dkim_private_key)) {
throw new TypeError(`${path}.dkim_private_key must be a non-empty string.`);
}
}
/**
* Normalises the base URL to ensure exactly one trailing slash.
*
* @param baseUrl - Optional base URL provided by the consumer.
* @returns The canonical base URL.
*/
function normalizeBaseUrl(baseUrl: string | undefined): string {
const normalized = (baseUrl ?? DEFAULT_BASE_URL).trim().replace(/\/?$/, "/");
return normalized;
}
/**
* Provides a high-level, DKIM-aware interface to the MailChannels `/send` endpoint.
*
* The client applies DKIM defaults, validates payloads, and forwards requests using the
* Fetch API available in Node.js 18+.
*
* @public
*/
export class MailChannelsClient {
private readonly apiKey: string;
private readonly baseUrl: string;
private readonly fetchImpl: typeof fetch;
private readonly defaultHeaders: Record<string, string>;
private readonly dkim: DkimConfig;
/**
* Creates a new MailChannels client instance.
*
* @param options - Configuration describing API credentials, DKIM defaults, and runtime hooks.
* @throws {TypeError} When required options (API key, DKIM fields, fetch implementation) are missing.
*/
constructor(options: MailChannelsClientOptions) {
if (!options || typeof options !== "object") {
throw new TypeError(
"`options` is required when constructing MailChannelsClient."
);
}
if (!options.apiKey || !options.apiKey.trim()) {
throw new TypeError("`apiKey` must be a non-empty string.");
}
const dkim = options.dkim;
if (!dkim || typeof dkim !== "object") {
throw new TypeError(
"`dkim` configuration is required when constructing MailChannelsClient."
);
}
const { domain, selector, privateKey } = dkim;
if (!hasText(domain)) {
throw new TypeError("`dkim.domain` must be a non-empty string.");
}
if (!hasText(selector)) {
throw new TypeError("`dkim.selector` must be a non-empty string.");
}
if (!hasText(privateKey)) {
throw new TypeError(
"`dkim.privateKey` must be a non-empty string containing the Base64-encoded key."
);
}
const fetchImpl = options.fetchImplementation ?? globalThis.fetch;
if (typeof fetchImpl !== "function") {
throw new TypeError(
"No Fetch API implementation available. Provide one via options.fetchImplementation."
);
}
this.apiKey = options.apiKey;
this.baseUrl = normalizeBaseUrl(options.baseUrl);
this.fetchImpl = fetchImpl;
this.defaultHeaders = options.defaultHeaders
? { ...options.defaultHeaders }
: {};
this.dkim = {
domain: domain.trim(),
selector: selector.trim(),
privateKey: privateKey.trim(),
};
}
/**
* Sends an email via the MailChannels `/send` endpoint.
*
* @param payload - Structured message body documented at
* https://docs.mailchannels.net/email-api/sending-email/email-structure.
* DKIM fields are automatically populated from the client defaults unless explicitly overridden.
* @param options - Additional request options, including dry-run support.
* @returns Parsed response metadata, including the HTTP status and any message identifier.
* @throws {TypeError} When validation fails before issuing the HTTP request.
* @throws {MailChannelsError} When the MailChannels API responds with a non-success status.
* The error contains HTTP diagnostics such as the status text, response headers, request
* identifier, retry hints, and either the parsed JSON body or raw response text.
*/
public async sendEmail(
payload: SendEmailRequest,
options: SendEmailOptions = {}
): Promise<SendEmailResponse> {
const normalizedPayload = this.applyDkimDefaults(payload);
validateSendRequest(normalizedPayload);
const requestUrl = new URL(SEND_PATH, this.baseUrl);
if (options.dryRun) {
requestUrl.searchParams.set("dry-run", "true");
}
const headers: Record<string, string> = {
"content-type": "application/json",
"X-Api-Key": this.apiKey,
...this.defaultHeaders,
};
if (options.idempotencyKey) {
headers["Idempotency-Key"] = options.idempotencyKey;
}
const response = await this.fetchImpl(requestUrl.toString(), {
method: "POST",
headers,
body: JSON.stringify(normalizedPayload),
signal: options.signal,
});
const rawText = await response.text();
const contentType = response.headers.get("content-type");
let parsedBody: unknown;
if (rawText.length > 0 && contentType?.includes("application/json")) {
try {
parsedBody = JSON.parse(rawText);
} catch {
// fall through to capturing raw text in the error below.
}
}
if (!response.ok) {
const statusText = hasText(response.statusText)
? response.statusText.trim()
: undefined;
const message = deriveErrorMessage(
response.status,
statusText,
parsedBody,
rawText
);
const details = parsedBody ?? (rawText.length > 0 ? rawText : undefined);
throw new MailChannelsError(message, {
status: response.status,
statusText,
requestId: extractRequestId(response.headers),
retryAfterSeconds: parseRetryAfterSeconds(response.headers),
headers: serializeHeaders(response.headers),
details,
});
}
let responseId: string | undefined;
let responseData: unknown;
if (parsedBody && typeof parsedBody === "object") {
const bodyAsRecord = parsedBody as Record<string, unknown>;
if (typeof bodyAsRecord.id === "string") {
responseId = bodyAsRecord.id;
}
if (Object.keys(bodyAsRecord).length > (responseId ? 1 : 0)) {
responseData = parsedBody;
}
} else if (rawText.length > 0) {
responseData = rawText;
}
const responsePayload: SendEmailResponse = {
status: response.status,
...(responseId ? { id: responseId } : {}),
...(responseData !== undefined ? { data: responseData } : {}),
};
return responsePayload;
}
/**
* Applies client-level DKIM defaults while preserving explicit overrides.
*
* @param payload - Original payload provided by the caller.
* @returns A new payload containing guaranteed DKIM fields.
*/
private applyDkimDefaults(payload: SendEmailRequest): SendEmailRequest {
const globalDomain = selectDkimValue(payload.dkim_domain, this.dkim.domain);
const globalSelector = selectDkimValue(
payload.dkim_selector,
this.dkim.selector
);
const globalPrivateKey = selectDkimValue(
payload.dkim_private_key,
this.dkim.privateKey
);
const personalizations = payload.personalizations.map((personalization) => {
const personalizationDomain = selectDkimValue(
personalization.dkim_domain,
globalDomain
);
const personalizationSelector = selectDkimValue(
personalization.dkim_selector,
globalSelector
);
const personalizationPrivateKey = selectDkimValue(
personalization.dkim_private_key,
globalPrivateKey
);
return {
...personalization,
dkim_domain: personalizationDomain,
dkim_selector: personalizationSelector,
dkim_private_key: personalizationPrivateKey,
};
});
return {
...payload,
personalizations,
dkim_domain: globalDomain,
dkim_selector: globalSelector,
dkim_private_key: globalPrivateKey,
};
}
}
/**
* Returns the first non-empty DKIM value between a candidate and fallback.
*
* @param value - Candidate value supplied by the caller.
* @param fallback - Default value used when the candidate is missing.
* @returns Trimmed DKIM value guaranteed to be non-empty.
*/
function selectDkimValue(value: string | undefined, fallback: string): string {
if (hasText(value)) {
return value.trim();
}
return fallback;
}
export type {
Attachment,
ContentBlock,
DkimConfig,
EmailAddress,
MailChannelsClientOptions,
Personalization,
SendEmailOptions,
SendEmailRequest,
SendEmailResponse,
};
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