To avoid the mistake of initializing a variable tonull, we can use like this: if (typeof variable === 'undefined' || variable === null) { // variable is undefined or null Solution 4: In JavaScript, a variable can be defined, but hold the valueundefined. ...
If you want to check whether a variable has been initialized or defined (i.e. test whether a variable has been declared and assigned a value) you can use the typeof operator.The most important reason of using the typeof operator is that it does not throw the ReferenceError if the ...
log("Age is not defined"); } 4. Use typeof for Safe Checks Prefer using the typeof operator to check if a variable is undefined, especially when the variable may be undeclared. This method is safe because it won’t throw an error if the variable is undeclared. Following is the sampl...
varv =1;functionf() {console.log(v); } f()// 1 上面的代码表明,函数f内部可以读取全局变量v。 在函数内部定义的变量,外部无法读取,称为“局部变量”(local variable)。 functionf(){varv =1; } v// ReferenceError: v is not defined 上面代码中,变量v...
The Number.isFinite() function checks if the variable is a number, but also checks if it's a finite value. Therefore, it returns false on numbers that are NaN, Infinity or -Infinity. Let's test it out on the variables we've defined above: > Number.isFinite(intVar); true > Number...
let age = 18; // Cannot redeclare block-scoped variable 'age'var age = 22; // Uncaught SyntaxError: Identifier 'age' has already been declared 因为这样var会hoist,其实就等价于: 代码语言:javascript 代码运行次数:0 运行 AI代码解释 var age;let age = 18;age = 22; 所以在使用let声明变量时,...
// assigning a value to the variable `x`foo(x,y);// calling function `foo` with parameters `x` and `y`obj.bar(3);// calling method `bar` of object `obj`// A conditional statementif(x===0){// Is `x` equal to zero?x=123;}// Defining function `baz` with parameters `a`...
test(); 这强制程序员养成好的习惯,变量需要“先声明再使用”,否则报错。 以下摘自MDN的关于let不在发生变量提升的描述 In ECMAScript 6,letdoes not hoistthe variable to the top of the block. If you reference a variable in a block before theletdeclaration for that variable is encountered, this res...
ES6 is not supported in Internet Explorer. JavaScript let Theletkeyword allows you to declare a variable with block scope. Example varx =10; // Here x is 10 { letx =2; // Here x is 2 } // Here x is 10 Try it Yourself » ...
A function is a piece of executable code that is defined by a JavaScript program or predefined by the JavaScript implementation. 3.1、方法的参数 1、js中有内部对象arguments,可以把他当成数组访问参数, 即使在定义方法的时候没有设置参数,但在使用方法是加入参数 ...