var lastname = "Hi"; if(typeof lastname !== "undefined") { alert("Hi. Variable is defined."); }. ... <看更多>

console.log(a); // 未宣告直接取值，Uncaught ReferenceError: a is not defined. null 是一種型態，代表 空值. 物件原型鍊的終點. ... <看更多>

undefined is a special keyword in JavaScript and `undefined !== not defined`. JS is a loosely typed ... ... <看更多>

An undefined variable is a variable that has been declared. Because we have not assigned it a value, the variable used the undefined as its initial value. ... <看更多>

Not a big deal to add is-defined function (I expect it to appear in ... I.e. in summary: instead of testing if a variable is defined just ... ... <看更多>

return (!value || value == undefined || value == "" || value.length == 0); ... is "string" (and thus non-null and not undefined), and if it is not empty. ... <看更多>