I recently used Crockford s JSLint to check some of my JavaScript code and it returned the following error

Question

Problem at line 1 character 1: Missing "use strict" statement.

I discovered after some searching that some individuals add the phrase "use strict"; to their JavaScript code. I added the statement, and the mistake ceased to occur. Sadly, Google did not provide any information on the background of this string statement. I'm sure it has to do with how the browser interprets the JavaScript, but I have no idea what the outcome might be.

What exactly does "use strict" mean, what does it suggest, and is it still applicable today?

Does the "use strict" string work with any of the current browsers, or is it just for the future?

Answer 1

"use strict" is a directive in JavaScript that enables strict mode in a script or a function. When strict mode is enabled, the JavaScript interpreter enforces stricter rules and generates more exceptions for common mistakes, making it easier to write reliable and maintainable code.

Here are some of the key behaviors enforced by strict mode:

1. Variables must be declared with var, let, or const before being used.
2. Assignments to undeclared variables are not allowed.
3. Deleting variables, functions, or function arguments is not allowed.
4. Duplicate parameter names in function declarations are not allowed.
5. Octal literals (e.g., 0123) are not allowed.
6. this is undefined in functions that are not methods or constructors.
7. The use of eval in the global scope or with direct assignments to eval is not allowed.
8. The use of reserved words (e.g., implements, interface, let, package, private, protected, public, static, yield) as variable or function names is not allowed.