No colon after property name in object declaration, is it valid? [duplicate]

Question

I entered this expression in the Firefox and Chrome Dev Console and I wonder why it is valid JavaScript:

var x = { a (b) {} };
console.log(x);

x is then set to an object with the property "a" containing a function called "a" with an argument identifier "b". How is this valid JavaScript syntax? The colon is missing after "a" and I do not understand the function definition.

Answer 1

This is ES6 / ES2015 syntactic sugar (Property shorthand). With ES6:

const obj = {
    a(b) { 
        // Shorthand method
        // `this` context is `obj`
    },
    c
};

is equal to

var obj = {
    a: function a(b) {

    },
    c: c
};

Answer 2

In JavaScript, when you write:

var x = { a (b) {} };

It will consider it as:

var x = { 
    a: function (b) {

     }
   }

For example, you can check this and it will clear your doubt:

var x = { a (b) { console.info('function called') } };
x.a(); 

This will call the function which is assigned to property a of object x.