How to fix Strict mode is not enabled in TypeScript

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This TypeScript configuration error occurs when the "strict" compiler option is not enabled in tsconfig.json. Strict mode provides stronger type checking and helps catch potential runtime errors during compilation. Enabling it improves code quality and prevents common type-related bugs.

What this error means

The "Strict mode is not enabled" warning or error indicates that TypeScript's comprehensive strict type checking features are disabled in your project configuration. TypeScript's strict mode is actually a collection of several individual strict checking flags that work together to provide maximum type safety. When strict mode is not enabled, TypeScript uses more permissive type checking rules, which can lead to: - Implicit "any" types being allowed - Less strict null checking - Weaker function type checking - Potential runtime errors that could have been caught at compile time This is typically not an error that stops compilation, but rather a configuration warning that suggests enabling stricter type checking for better code quality and error prevention. Many development teams and projects require strict mode to be enabled to maintain high code quality standards.

How to fix "Strict mode is not enabled"

1Check your current tsconfig.json configuration

First, examine your tsconfig.json file to see if strict mode is enabled:

json

// tsconfig.json
{
  "compilerOptions": {
    // Look for the "strict" property
    "strict": true  // Should be true to enable strict mode
  }
}

If the "strict" property is missing or set to false, that's the cause of the warning.

2Enable strict mode in tsconfig.json

Update your tsconfig.json to enable strict mode:

json

{
  "compilerOptions": {
    "strict": true,
    // Other compiler options...
    "target": "es2020",
    "module": "commonjs",
    "outDir": "./dist"
  }
}

Setting "strict": true enables all strict type-checking options at once.

3Fix any new type errors that appear

After enabling strict mode, you may see new type errors. Common fixes include:

1. Add explicit type annotations for variables with implicit 'any' type:

typescript

// Before: implicit any
const myVar = getValue();

// After: explicit type
const myVar: string = getValue();

2. Handle null/undefined properly with strict null checks:

typescript

// Before: potential runtime error
const length = someString.length;

// After: null check
const length = someString?.length ?? 0;

3. Fix function type mismatches that strict mode catches.

4Enable individual strict options if needed

If enabling all strict checks at once causes too many errors, you can enable them individually:

json

{
  "compilerOptions": {
    "strict": false,  // Disable the umbrella flag
    // Enable specific strict checks
    "noImplicitAny": true,
    "strictNullChecks": true,
    "strictFunctionTypes": true,
    "strictBindCallApply": true,
    "strictPropertyInitialization": true,
    "noImplicitThis": true,
    "alwaysStrict": true
  }
}

This lets you adopt strict mode gradually by fixing one category of errors at a time.

5Verify strict mode is working

Create a test file to verify strict mode is enabled:

typescript

// test-strict.ts
// This should cause an error with strict mode enabled
const implicitAny = (x) => x + 1;  // Error: Parameter 'x' implicitly has 'any' type

// This should also cause an error
let potentiallyNull: string | null = null;
const length = potentiallyNull.length;  // Error: Object is possibly 'null'

Run tsc --noEmit to check for type errors. If you see errors for these cases, strict mode is working.

6Update build and CI configurations

Ensure your build tools and CI/CD pipeline respect the tsconfig.json changes:

1. Update package.json scripts:

json

{
  "scripts": {
    "build": "tsc",
    "type-check": "tsc --noEmit"
  }
}

2. Update CI configuration (GitHub Actions, GitLab CI, etc.) to run type checking:

yaml

# GitHub Actions example
- name: Type check
  run: npm run type-check

3. Update IDE/editor settings to use the project's tsconfig.json.

How to fix Strict mode is not enabled in TypeScript

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How to fix "Strict mode is not enabled"

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