Javascript undefined vs null

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undefined is the lack of a type and value, and null is the lack of a value.

Furthermore, if you'redoing simple '==' comparisons, you're right, they come out the same. But try ===, which compares both type and value, and you'll notice the difference.

The question is,"why is there an undefined value in JS". Major places where it is used:

1. when you declare 'var x;' but don't assign to it, x holds undefined;

2. when your function gets fewer arguments than it declares;

3. when you access a non-existent object property.

var a = undefined;var a = null;if (!undefined)     console.log('undefined is false');// undefined is falseif (!null)     console.log('null is false');// null is falseundefined == null// trueNumber(null)// 05 + null// 5Number(undefined)// NaN5 + undefined// NaN//null是一个表示"无"的对象，转为数值时为0；undefined是一个表示"无"的原始值，转为数值时为NaN。//null表示"没有对象"，即该处不应该有值//（1） 作为函数的参数，表示该函数的参数不是对象。//（2） 作为对象原型链的终点。Object.getPrototypeOf(Object.prototype)// null//undefined表示"缺少值"，就是此处应该有一个值，但是还没有定义//（1）变量被声明了，但没有赋值时，就等于undefined。//（2) 调用函数时，应该提供的参数没有提供，该参数等于undefined。//（3）对象没有赋值的属性，该属性的值为undefined。//（4）函数没有返回值时，默认返回undefined。var i;i // undefinedfunction f(x){console.log(x)}f() // undefinedvar  o = new Object();o.p // undefinedvar x = f();x // undefined