javascript - How might I be able to use Array.prototype.forEach on an object? - Stack Overflow

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I'm just trying to make a little framework for learning purposes, and I want to be able to use Array.prototype.forEach on objects, this is what I have right now,

var data = { test: 'test' };

Array.prototype.forEach(data, function(key value) {
    console.log(key);
});

But I get an error, I'm sure you guys can see why, but I can not :) So any help would be great thanks :)

I'm just trying to make a little framework for learning purposes, and I want to be able to use Array.prototype.forEach on objects, this is what I have right now,

var data = { test: 'test' };

Array.prototype.forEach(data, function(key value) {
    console.log(key);
});

But I get an error, I'm sure you guys can see why, but I can not :) So any help would be great thanks :)

• javascript
• prototype

Share Improve this question Follow edited May 13, 2013 at 16:00 Ben McCormick 25.7k1212 gold badges5555 silver badges7171 bronze badges asked May 13, 2013 at 15:57 DatsikDatsik 14.8k1515 gold badges8585 silver badges128128 bronze badges 1

• I'd assume the "Why?" is because an object isn't an array. You also have a syntax error in the parameters in your anonymous function. – Anthony Grist Commented May 13, 2013 at 15:59

Add a ment  | 

6 Answers 6

Sorted by: Reset to default 5

Objects are not arrays, and don't have access to the array prototype. You can just loop over the properties.

for(var key in data){
      console.log(key) //or data[key] if you want the values
   }

with regards to the jQuery ment below, it appears they use loops internally for their "each" function. From the source:

// args is for internal usage only
each: function( obj, callback, args ) {
    var value,
        i = 0,
        length = obj.length,
        isArray = isArraylike( obj );

    if ( args ) {
        if ( isArray ) {
            for ( ; i < length; i++ ) {
                value = callback.apply( obj[ i ], args );

                if ( value === false ) {
                    break;
                }
            }
        } else {
            for ( i in obj ) {
                value = callback.apply( obj[ i ], args );

                if ( value === false ) {
                    break;
                }
            }
        }

    // A special, fast, case for the most mon use of each
    } else {
        if ( isArray ) {
            for ( ; i < length; i++ ) {
                value = callback.call( obj[ i ], i, obj[ i ] );

                if ( value === false ) {
                    break;
                }
            }
        } else {
            for ( i in obj ) {
                value = callback.call( obj[ i ], i, obj[ i ] );

                if ( value === false ) {
                    break;
                }
            }
        }
    }

    return obj;
},

As mentioned above, Object and Array are different things. You can create a function that will do this for you, by using "call" for invocation:

Object.prototype.forEach = function(func, context) {
    var value;
    context = context || this;  //apply the function to 'this' by default
    for (key in this) {
        if (this.hasOwnProperty(key)) {  //to rule out inherited properties
            value = this[key];
            func.call(context, key, value);
        }
    }    
};

And use it like this:

obj = { "test1": 1, "test2": 2 };
obj.forEach(function(key, val) { 
    console.log(key, val); 
});

Do it like this:

Object.keys(data).forEach(function(key) {
    console.log(data[key])
});

You can shim Object.keys() for old browsers if needed.

---

If you prefer, you can make a wrapper function for this operation:

function objEach(obj, fn) {
    Object.keys(obj).forEach(function(key) {
        fn(obj[key], key)
    })
    // or use for-in
}

Then use it like this:

objEach(data, function(val, key) {
    console.log(key, val)
})

With ES2017, there is Object.entries():

const obj = { 0:'value1', key:'value2' };

Object.entries(obj).forEach(([key, value]) => {
    console.log(key, value);
});

For ES5, there is Object.keys():

const obj = { 0:'value1', key:"value2" };

Object.keys(obj).forEach(key => {
    console.log(key, obj[key]);
});

If your object is Array-like (meaning that it has numerical keys and a length), there is ES2015's Array.from():

const obj = { 0:'value1', 1:'value2', length:2 };

Array.from(obj).forEach((element, i) => {
    console.log(i, element);
});

You can iterate through the objects keys by for(key in data) and get each value by data[key]. I.e.

for(key in object) {
    console.log(key, data[key]);
}

Try this:

obj = {}; // Some object
obj.forEach = function(fn) {
    Object.keys(this).forEach((k)=> {
        if (typeof(this[k]) != 'function') 
            fn(this[k]);})
        };

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