TA貢獻(xiàn)1946條經(jīng)驗(yàn) 獲得超4個(gè)贊
alert(typeof(null)); // objectalert(typeof(undefined)); // undefinedalert(null !== undefined) //truealert(null == undefined) //true
object == nullif ( !object ).
! Boolean(object)!
Boolean(null)!false === true.
alert(Boolean(null)) //falsealert(Boolean(0)) //falsealert(Boolean("")) //false
TA貢獻(xiàn)1828條經(jīng)驗(yàn) 獲得超6個(gè)贊
nulltypeof null"object"
nullundefined
undefined
> var noValueYet;> console.log(noValueYet);undefined> function foo(x) { console.log(x) }> foo()undefined> var obj = {};
> console.log(obj.unknownProperty)undefined> unknownVariableReferenceError: unknownVariable is not defined
null
console.log(typeof unknownVariable === "undefined"); // truevar foo;console.log(typeof foo === "undefined"); // trueconsole.log(foo === undefined); // truevar bar = null;console.log(bar === null); // true
x == nullnullundefined:
> null == nulltrue> undefined == nulltrue
trueif
function foo(param) {
if (param) {
// ...
}}function foo(param) {
if (! param) param = "abc";}function foo(param) {
// || returns first operand that can't be converted to false
param = param || "abc";}falseundefined0).
undefined, null
false
+0, -0, NaN
""
Booleannew):
> Boolean(null)false> Boolean("")false> Boolean(3-3)false> Boolean({})true> Boolean([])true舉報(bào)