Declaring a JavaScript Variable Creating a variable in JavaScript is called "declaring" a variable. You declare a JavaScript variable with thevaror theletkeyword: varcarName; or: letcarName; After the declaration, the variable has no value (technically it isundefined). ...
// Does x have a value? if (x !== undefined && x !== null) { ... } // Is x a non-value? if (x === undefined || x === null) { ... } 另一种方法是利用undefined和null都被视为false的事实(请参阅真值和假值): // Does x have a value (is it truthy)? if (x) { ...
If a scope declares a variable that has the same name as one in a surrounding scope, access to the outer variable is blocked in the inner scope and all scopes nested inside it. Changes to the inner variable do not affect the outer variable, which is accessible again after the inner scope...
Creating a variable in JavaScript is called "declaring" a variable. You declare a JavaScript variable with thevarkeyword: varcarName; After the declaration, the variable has no value. (Technically it has the value ofundefined) Toassigna value to the variable, use the equal sign: ...
Declaring a Variable To declare a variable in JavaScript, any of these three keywords can be used along with a variable name: var is used in pre-ES6 versions of JavaScript. It is function scoped. let is the preferred way to declare a variable when it can be reassigned. It is block scop...
Declaring a variable withconstis similar toletwhen it comes toBlock Scope. The x declared in the block, in this example, is not the same as the x declared outside the block: Example constx =10; // Here x is 10 { constx =2; ...
// `if` statementif (answer !== 42) { answer = 42 }// `for` is a statementfor (;;) { console.log('Hello, World'); }// Declaring a variable is a statementlet answer = 42 函数参数 当您调用 JavaScript 函数时，所有参数都必须是表达式，而不是语句。function print(v) { console....
When we usually declare variables, I will find that a variable can only store one value. If we want to store more than one value, then the name of our previous variable can't be realized. At this time, we can use arrays to store. Instead of declaring a variable name for each element...
// Two slashes start single-line commentsvarx;// declaring a variablex=3+y;// 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...
Here are a few examplesof basic syntax: // Two slashes start single-linecommentsvarx;// declaring a variablex=3+y;// 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...