null in this code of javascript?

Question

    Array.prototype.forEach = function(callback, context) {
        for (var i = 0; i < this.length; i++) {
            callback.call(context || null, this[i], i, this);
        }
    };

    ["a", "b", "c"].forEach(function(value, index, array) {
        assert(value,
                "Is in position " + index + " out of " +
                        (array.length - 1));
    });

I don't fully understand why null is used here. I guess when I use the invoke foreach, if I miss the context parameter, it will replace it with null? Will callback.call(context || null, this[i], i, this) execute? Can someone explain this for me?

Blender · Accepted Answer · 2013-08-07 02:33:24Z

1

It shouldn't actually be there. undefined and null have the same effect when passed as the this argument to Function.prototype.call (the this argument of the function is set to undefined).

answered Aug 7, 2013 at 2:33

Blender

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2 Comments

joe smith Over a year ago

so even i pass null or undefined to Function.prototype.call, the Function.prototype.call still can execute ?

Blender Over a year ago

@joesmith: The this argument can take any value. In non-strict mode, if you pass null or undefined as the this argument, this will be undefined in the function.

SaganRitual · Accepted Answer · 2013-08-07 02:25:47Z

1

If you pass a falsy value for 'context', (context || null) will result in a null. JS will pass null as the first parameter into callback.call(). The first parameter is the this for the callback function.

answered Aug 7, 2013 at 2:25

SaganRitual

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