Function mergeWith

Alias

  • mergeWith<T, S>(target, source, merge): T & S

  • Merges the properties of the source object into the target object.

    You can provide a custom merge function to control how properties are merged. It should return the value to be set in the target object.

    If it returns undefined, a default deep merge will be applied for arrays and objects:

    • If a property in the source object is an array or an object and the corresponding property in the target object is also an array or object, they will be merged.
    • If a property in the source object is undefined, it will not overwrite a defined property in the target object.

    Note that this function mutates the target object.

    Type Parameters

    • T extends Record<PropertyKey, any>

      Type of the target object.

    • S extends Record<PropertyKey, any>

      Type of the source object.

    Parameters

    • target: T

      The target object into which the source object properties will be merged. This object is modified in place.

    • source: S

      The source object whose properties will be merged into the target object.

    • merge: ((targetValue: any, sourceValue: any, key: string, target: T, source: S) => any)

      A custom merge function that defines how properties should be combined. It receives the following arguments:

      • targetValue: The current value of the property in the target object.
      • sourceValue: The value of the property in the source object.
      • key: The key of the property being merged.
      • target: The target object.
      • source: The source object.
        • (targetValue, sourceValue, key, target, source): any

        • Parameters

          • targetValue: any
          • sourceValue: any
          • key: string
          • target: T
          • source: S

          Returns any

    Returns T & S

    The updated target object with properties from the source object merged in.

    Example

    const target = { a: 1, b: 2 };
    const source = { b: 3, c: 4 };

    mergeWith(target, source, (targetValue, sourceValue) => {
      if (typeof targetValue === 'number' && typeof sourceValue === 'number') {
        return targetValue + sourceValue;
      }
    });
    // Returns { a: 1, b: 5, c: 4 }

    Example

    const target = { a: [1], b: [2] };
    const source = { a: [3], b: [4] };

    const result = mergeWith(target, source, (objValue, srcValue) => {
      if (Array.isArray(objValue)) {
        return objValue.concat(srcValue);
      }
    });

    expect(result).toEqual({ a: [1, 3], b: [2, 4] });

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