typeof
typeof function(){} 'function' typeof {} 'object' typeof [] 'object' typeof '' 'string' typeof 0 'number' typeof true 'boolean' typeof null 'object' typeof undefined 'undefined' typeof [].length 'number' typeof {}.length 'undefined'
Boolean(function(){}) true Boolean({}) true Boolean([]) true Boolean(null) false Boolean(undefined) false Boolean('') false Boolean(' ') true Boolean(0) false Boolean(1) true Boolean(false) false Boolean(true) true