javascript - JavaScript检查变量是否存在（已定义/初始化）(JavaScript check if variable exists (is defined/initialized))

Question

Which method of checking if a variable has been initialized is better/correct?

(哪种方法检查变量是否已初始化是更好/正确的方法？)

(Assuming the variable could hold anything (string, int, object, function, etc.))

(（假设变量可以容纳任何内容（字符串，整数，对象，函数等）。)

if (elem) { // or !elem

or

(要么)

if (typeof(elem) !== 'undefined') {

or

(要么)

if (elem != null) {

  ask by Samuel Liew translate from so

Answer 1

You want the typeof operator .

(您需要typeof运算符 。)

Specifically:

(特别：)

if (typeof variable !== 'undefined') {
    // the variable is defined
}