Differences between let and var variables defined in js, jsletvar Variables

Differences between let and var variables defined in js, jsletvar Variables

Strict javascript Mode

The first touch with the let keyword has a very important concept that is "javascript strict mode". For example, if the following code is run, an error is reported:

let hello = 'hello world.';console.log(hello);

The error message is as follows:

let hello = 'hello world.';^^^SyntaxError: Block-scoped declarations (let, const, function, class) not yet supported outside strict mode ...

The solution is to add the "javascript strict mode" statement in the file header:

'use strict';let hello = 'hello world.';console.log(hello);

Similarities and differences between let and var keywords

No value assigned after declaration, same performance

'Use strict '; (function () {var varTest; let letTest; console. log (varTest); // output undefined console. log (letTest); // output undefined }());

Using undeclared variables is different:

(Function () {console. log (varTest); // output undefined (note that you need to comment out the following line before running) console. log (letTest); // the following error occurs: ReferenceError: letTest is not defined var varTest = 'test var OK. '; let letTest = 'test let OK. ';}());

Repeated declarations of the same variable are different:

'Use strict '; (function () {var varTest = 'test var OK. '; let letTest = 'test let OK. '; var varTest = 'vartest changed. '; let letTest = 'lettest changed. '; // directly reports the following error: SyntaxError: Identifier 'lettest' has already been declared console. log (varTest); // output varTest changed. (note that you need to comment out the repeated declaration of the letTest variable above before running) console. log (letTest );}());

Variable scope, different performance

'Use strict '; (function () {var varTest = 'test var OK. '; let letTest = 'test let OK. '; {var varTest = 'vartest changed. '; let letTest = 'lettest changed. ';} console. log (varTest); // output "varTest changed. ", the varTest variable declared in the internal" {} "overwrites the external letTest declaration console. log (letTest); // output "test let OK. ", the letTest declared in the internal" {} "is not the same variable as the external letTest }());

Summary

The above section describes the differences between let and var variables in Javascript. I hope it will be helpful to you. If you have any questions, please leave a message for me, the editor will reply to you in a timely manner. Thank you very much for your support for the help House website!