For..In loops in JavaScript - key value pairs

No one has mentioned Object.keys so I'll mention it.

Object.keys(obj).forEach(function (key) {
   // do something with obj[key]
});

for (var k in target){
    if (target.hasOwnProperty(k)) {
         alert("Key is " + k + ", value is " + target[k]);
    }
}

hasOwnProperty is used to check if your target really has that property, rather than having inherited it from its prototype. A bit simpler would be:

for (var k in target){
    if (typeof target[k] !== 'function') {
         alert("Key is " + k + ", value is" + target[k]);
    }
}

It just checks that k is not a method (as if target is array you'll get a lot of methods alerted, e.g. indexOf, push, pop,etc.)

for...in will work for you.

for( var key in obj ) {
  var value = obj[key];
}

In modern JavaScript you can also do this:

for ( const [key,value] of Object.entries( obj ) ) {

}

If you can use ES6 natively or with Babel (js compiler) then you could do the following:

const test = {a: 1, b: 2, c: 3};

for (const [key, value] of Object.entries(test)) {
  console.log(key, value);
}

Which will print out this output:

a 1
b 2
c 3

The Object.entries() method returns an array of a given object's own enumerable property [key, value] pairs, in the same order as that provided by a for...in loop (the difference being that a for-in loop enumerates properties in the prototype chain as well).

• Object.entries documentation
• for...of documentation
• Destructuring assignment documentation
• Enumerability and ownership of properties documentation