JavaScript Type Conversions 

JavaScript Primitives yes, js has primitives 

1. undefined 2. null 3. number 4. string 5. boolean 

1. undefined 2. null yep, is not an object 3. number 4. string 5. boolean 

JavaScript Binary Operators we are going to address only - and + 

On doubt check the spec 

http://www.ecma-international.org/ecma-262/5.1/#sec-11.6.1 http://www.ecma-international.org/ecma-262/5.1/#sec-11.6.2 

+ Operator 

If lprim or rprim are strings, then concatenate lprim and rprim and return the result 

- Operator 

ToNumber(lprim) - ToNumber(rprim) 

Examples 

var a, b; a = "bla"; b = "ble"; a + b; //=> "blable" a - b; //=> "NaN" a = "5"; b = "4"; a + b; //=> "54" a - b; //=> 1 

var obj = { valueOf: function valueOf() { console.log("valueOf"); return {}; // not a primitive }, toString: function toString() { console.log("toString"); return {}; // not a primitive } }; obj - 1; // valueOf // toString // error obj + 1; // valueOf // toString // error 

var func = function () { console.log('exec'); return { valueOf: function valueOf() { console.log("valueOf"); return {}; // not a primitive }, toString: function toString() { console.log("toString"); return {}; // not a primitive } }; }; func() + 1; // exec // valueOf // toString // error 

{} + [] //=> +[] == 0 [] + {} //=> '' + '[object Object]' == '[object Object]' [] - {} //=> 0 - NaN == NaN {} - [] //=> -[] == -0 

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