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I'm not familiar with ECMAScript 6 yet. I've just cloned the React Starter Kit repo, which uses ES6 for application code. I was surprised to see that the linter is configured to forbid occurences of the use strict directive, which I thought was recommended in pre-ES6 JavaScript. So what's the point?
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Strict Mode(“use strict”) helps identify common issues (or “bad” parts) and also helps with “securing” JavaScript. In ES5, the Strict Mode is optional but in ES6, it's needed for many ES6 features.
If you have such an unrestrictedly typed code, that is used variables without declaring. One variable declared within some function/scope and used from somewhere else(it will be undeclared there) and you can't rewrite/change them, then you should not go for "use strict;" mode because it will break the code.
Is use strict necessary? The strict mode is no longer required since the release of ES2015, which fixes most of JavaScript's confusing behavior with a more robust syntax. It's just good to know because you might find it in legacy projects that still used it.
First, all of your code absolutely should be run in strict mode. Core modern javascript functionality is changed (see . call() and apply()) or disfigured (silent Errors) by executing code outside of strict mode.
ES6 modules are always in strict mode. To quote the relevant part of the spec:
10.2.1 Strict Mode Code
An ECMAScript Script syntactic unit may be processed using either unrestricted or strict mode syntax and semantics. Code is interpreted as strict mode code in the following situations:
- Global code is strict mode code if it begins with a Directive Prologue that contains a Use Strict Directive (see 14.1.1).
- Module code is always strict mode code.
- All parts of a ClassDeclaration or a ClassExpression are strict mode code.
- Eval code is strict mode code if it begins with a Directive Prologue that contains a Use Strict Directive or if the call to eval is a direct eval (see 12.3.4.1) that is contained in strict mode code.
- Function code is strict mode code if the associated FunctionDeclaration, FunctionExpression, GeneratorDeclaration, GeneratorExpression, MethodDefinition, or ArrowFunction is contained in strict mode code or if the code that produces the value of the function’s [[ECMAScriptCode]] internal slot begins with a Directive Prologue that contains a Use Strict Directive.
- Function code that is supplied as the arguments to the built-in Function and Generator constructors is strict mode code if the last argument is a String that when processed is a FunctionBody that begins with a Directive Prologue that contains a Use Strict Directive.
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