The following in a Javascript console:
var a = {'foo': []}; var b = {}; for (var key in a) { b[key] = a[key]; } a['foo'].push(1); console.log(b); Yields:
Object foo=[1]
I want to make a copy by value in b of each array for each key in a. Is there an easier way?
You could make a "clone" function that creates a new object, based on the original object constructor, and then clone that original object properties also if they are objects:
function clone(obj){ if(typeof(obj) != 'object' && obj != null) return obj; // return the value itself if isn't an object // or null, since typeof null == 'object'; var temp = new obj.constructor(); for(var key in obj) temp[key] = clone(obj[key]); return temp; } var a = {'foo': []}; var b = clone(a); a['foo'].push(1); console.log(b); // Object foo=[0] 
Object.prototype.foo = function () {}; will create infinite recursion.
obj.constructor() is going to die on constructor functions (temp will be undefined), and potentially clobber globals.new operator while I think on something better...This is called Deep Copy. You can find examples in:
Easier way:
var a = {'foo': []}; var b = a; a['foo'].push(1); console.log(b); Output is the same.
Edit:
var a = {'foo': []}; var b = {}; for (var key in a) { if (a.hasOwnProperty(key)) { b[key] = []; for (var i = 0; i < a[key].length; i += 1) { b[key][i] = a[key][i]; } } } a['foo'].push(1); console.log(b); 
a[key].slice(0).As this will add support for deep-copying arrays in your code:
var a = {'foo': []}; var b = {}; for (var key in a) { if (Object.prototype.toString.call(b[key]) === "[object Array]") { b[key] = a[key].slice(0); } else { b[key] = a[key]; } } a['foo'].push(1); console.log(b); 
console.logfunction to work.