support-nested-matching-in-custom-jest-matchers.md

Support Nested Matching In Custom Jest Matchers

A custom Jest matcher can use standard JavaScript operations to evaluate if the given value(s) should pass or not.

expect.extend({
  toContainValue(receivedArray, containedValue) {
    const pass =
      receivedArray.some(value => value === containedValue);

    // return formatted pass/not-pass objects with messages
    return { ... }
  }
});

This approach alone doesn't support the power of Jest's nested matchers. Consider trying to use this like so:

expect(['a', 2, true]).toContainValue(expect.any(Number));

This would fail, even though there is a value in there that matches any(Number).

Jest ships with some Jasmine utilities that you can use, just as Jest does internally, to perform nested matching:

const { equals } = require("expect/build/jasmineUtils");

expect.extend({
  toContainValue(receivedArray, containedValue) {
    const pass =
      receivedArray.some(value => equals(value, containedValue));

    // return formatted pass/not-pass objects with messages
    return { ... }
  }
});

That equals utility knows how to compare raw values like integers, booleans, and even whole objects against nested expect matchers.

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