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Strict Mode Explained

The opt-in stricter variant of JavaScript that catches silent errors, disables risky features, and is the default inside ES modules and classes.

Strict mode is a stricter variant of the language that turns silent failures into errors.

Sloppy mode is like a lenient proofreader who silently 'fixes' your typos without telling you; strict mode is an editor who stops and flags every mistake immediately so you actually learn about it instead of shipping it.

Key Concepts

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    'use strict';          // at the top of a file or a function
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It is the default in ES modules and inside classes. Anything in an import/export file, and any class body, is already strict. The directive is only needed in scripts and in non-module code.
importexport
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What it changes, and why each matters.Undeclared assignment throws.
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    x = 5;        // sloppy: creates a global. strict: ReferenceError
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A typo in a variable name used to create a new global and continue. Now it fails at the line that is wrong.
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Assigning to a read-only property throws rather than being ignored:
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    const o = Object.freeze({ a: 1 });
    o.a = 2;      // sloppy: silently nothing. strict: TypeError
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this is undefined in a plain function call.
thisundefined
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    function f() { console.log(this); }
    f();          // sloppy: the global object. strict: undefined
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This is the one that changes behaviour most visibly, and it is an improvement — a method called without its object now fails instead of quietly writing to globals.
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Duplicate parameter names are a syntax error, and octal literals like 0755 are rejected.
0755
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delete on a variable throws rather than returning false.
delete
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Why it exists rather than just being the language. Backwards compatibility. Making these changes unconditionally would have broken the web, so they were made opt-in — and then made the default in the two places that were new enough to afford it.
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The practical position. Write modules and you are in strict mode without doing anything. In older scripts, add the directive at the top of the file rather than per function.
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One caution when adding it to existing code. A file that relied on sloppy behaviour — an implicit global, a this that defaulted to the window object — will break. That is the point, and it is still worth doing per file rather than all at once.
this
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What the interviewer is probing.1. "What does strict mode change?" Probing: the specifics. Stalls: "It is stricter." *Moves up:* assigning to an undeclared variable throws, this is undefined rather than the global in plain functions, duplicate parameters are errors, and silent failures become thrown ones.
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2. "Where is strict mode automatic?" Probing: where it already applies. Stalls: "Only if you add the directive." Moves up: ES modules and class bodies are always strict, so most modern code already runs under it.
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3. "Why does strict mode help performance?" Probing: the engine argument. Stalls: "It does not." Moves up: it removes dynamic behaviours like with, letting the engine make stronger assumptions.
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4. "What breaks when you enable it on old code?" Probing: migration risk. Stalls: "Nothing." Moves up: accidental globals and functions relying on this defaulting to the global object.
Use cases:fundamentalsbest-practicessyntax