typeof能夠返回以下某個字符串

1. “undefined” — 假如這個值未定義
2. “boolean” — 假如這個值是布爾值
3. “string” — 假如這個值是字符串
4. “number” — 假如這個值是數值
5. “object” — 假如這個值是對象或許null
6. “function” — 假如這個值是函數

須要注重的幾種狀況

1. typeof Infinity === ‘number’;
2. typeof NaN === ‘number’; // 只管NaN是”Not-A-Number”的縮寫
3. typeof (typeof 1) === ‘string’; // typeof老是返回一個字符串
4. typeof undefined === ‘undefined’;
5. typeof declaredButUndefinedVariable === ‘undefined’;
6. typeof undeclaredVariable === ‘undefined’;

辨別數組,一般對象

1. typeof [1, 2, 4] === ‘object’;
2. typeof class C{} === ‘function’
3. typeof Math.sin === ‘function’;
4. typeof null === ‘object’; // 從一最先湧現JavaScript就是如許的

typeof為number舉例

1. typeof 37 === ‘number’;
2. typeof 3.14 === ‘number’;
3. typeof Math.LN2 === ‘number’;
4. typeof Infinity === ‘number’;
5. typeof NaN === ‘number’;
6. typeof Number(1) === ‘number’; // 不要運用這類情勢!

typeof為string舉例

1. typeof “” === ‘string’;
2. typeof “bla” === ‘string’;
3. typeof (typeof 1) === ‘string’; // typeof老是返回一個字符串
4. typeof String(“abc”) === ‘string’; // 不要運用這類情勢!

typeof為Booleans舉例

1. typeof true === ‘boolean’;
2. typeof false === ‘boolean’;
3. typeof Boolean(true) === ‘boolean’; // 不要運用這類情勢!

typeof為Symbols舉例

1. typeof Symbol() === ‘symbol’;
2. typeof Symbol(‘foo’) === ‘symbol’;
3. typeof Symbol.iterator === ‘symbol’;

typeof為Undefined舉例

1. typeof undefined === ‘undefined’;
2. typeof declaredButUndefinedVariable === ‘undefined’;
3. typeof undeclaredVariable === ‘undefined’;

typeof為Objects舉例

1. typeof {a:1} === ‘object’;
2. typeof [1, 2, 4] === ‘object’;
3. typeof new Date() === ‘object’;
4. typeof null === ‘object’;

typeof為function舉例

1. typeof function(){} === ‘function’;
2. typeof class C{} === ‘function’
3. typeof Math.sin === ‘function’;
4. typeof new Function() === ‘function’;

發起不要運用的

1. typeof new Boolean(true) === ‘object’;
2. typeof new Number(1) === ‘object’;
3. typeof new String(“abc”) === ‘object’;

參考文檔：MDN-typeof操作符