TypeScript 6.0 enforces strict mode and drops ES5 support for cleaner code

TypeScript 6.0 does not tiptoe around breaking changes. Strict mode is now enabled by default, ES5 as a target is gone, and module resolution defaults to ESNext, not CommonJS. If you have not explicitly set strict: true or have codebases still targeting ES5, this is not a routine upgrade. This release is the gatekeeper for the upcoming Go-native TypeScript 7.0—and if you don’t adapt quickly, your CI pipeline could grind to a halt when your dependencies move first.

This post is a hands-on migration map for the TypeScript 6.0 strict mode migration and the ES5 drop. We'll identify the new breaking behaviors, why they land now, and how you can upgrade without taking prod down for a week.

What are the major breaking changes in TypeScript 6.0?

There are three primary breaking changes in TypeScript 6.0, and each affects core assumptions around type safety, compat targets, and module wiring.

1. Strict mode (strict: true) is now the default.
Before 6.0, only teams who explicitly set "strict": true in their tsconfig.json opted into the full suite of type safety checks. That left roughly 70% of projects (quoted from the article) relying on legacy defaults that quietly allowed unsafe or error-prone patterns. Now, upgrading TypeScript automatically enables strict mode unless you opt out.

2. ES5 and ES3 as compiler targets are dropped.
TypeScript 6.0 no longer emits output in ES5 or ES3. The default output target moves to es2025, with annual updates planned. If your project targets IE or other ancient browsers, you'll need to add a separate transpilation pass—TypeScript won’t help.

3. Module resolution switches from CommonJS to ESNext by default.
The default module resolution algorithm is now ESNext, not CommonJS. This is a silent footgun for any Node.js project with CommonJS dependencies, or custom import patterns predicated on CJS semantics. Upgrade without reviewing this, and your imports or bundler may break.

Summed up, TypeScript 6.0 is not a gentle, opts-in-if-you-want migration. Dependencies will themselves upgrade and break your build unless you’re ahead of it. Read the official Medium release for concrete examples.

Why does TypeScript 6.0 drop ES5 and enable strict mode by default?

TypeScript's maintainers have clear reasons for breaking so many working setups in this release—and they're right to.

Legacy support imposes real costs.
Compatibility layers for deprecated JS like ES5/ES3 have cluttered TypeScript internals since 4.x. Removing them enables a leaner codebase and sets the groundwork for the 7.0 Go-native rewrite.

Strict mode is about more than safety—it’s about real bugs.
Strict mode is not a style preference. Enabling it turns on eight key type system checks (exact list omitted in the source, but includes noImplicitAny, strictNullChecks, etc.). These detect silent type errors—problems that ship as real bugs when unchecked.

ES5 removal is about modernizing, not spite.
Dropping ES5 means TypeScript no longer complicates its own codegen for the sake of 0.5% of web users still clinging to Internet Explorer or similar. If you still need to support those, you must own the Babel (or other) pipeline explicitly.

Net benefit: a smaller, faster compiler, more reliable typing by default, and a modern-out-of-the-box target for everyone. Painful now, but an obvious win over time.

How to assess if your codebase will break on TypeScript 6.0 upgrade

Don’t wait for CI to surprise you. Assess your risk before any version bump.

Check your tsconfig.json for an explicit strict setting.
If your config omits or sets "strict": false, the upgrade will drag strict mode in by default, surfacing potentially hundreds of new type errors.

// If absent or set to false, strict is now true by default in 6.0
{
  "compilerOptions": {
    // "strict": false,
  }
}

Review your target.
Are you still targeting ES5 or ES3?

// Will break in TS 6.0; ES5 and ES3 targets are gone
{
  "compilerOptions": {
    "target": "es5"
  }
}

Examine module resolution.
TypeScript 6.0’s default is now module: "esnext". This changes how imports and exports are resolved. If you depend on commonjs semantics (dynamic require, __dirname, etc.), fail to set this correctly and your bundler or app may throw import errors.

// Old default (will now break certain Node setups)
{
  "compilerOptions": {
    "module": "commonjs"
  }
}

Use codebase audits and linting.
Automate the detection:

grep strict tsconfig.json         # Find all explicit strict flags
grep target tsconfig.json         # Identify legacy ES5/3 targets
grep module tsconfig.json         # Audit module strategy

You can also use TS’s own CLI to preview issues:

npx tsc --noEmit --strict   # Run strict checks without emitting JS

Takeaway: any missing or legacy settings are now breaking defaults, not theoretical risks. Audit now, not after failures pile up.

Step-by-step migration guide to TypeScript 6.0 strict mode and ES5 removal

A chaotic all-at-once migration is the fastest route to CI downtime. The right way is incremental, controlled, and explicit.

1. enable strict mode and fix surfaced errors.
Before upgrading, set "strict": true in your current TS version.

{
  "compilerOptions": {
    "strict": true
  }
}

Run a strict-only check to get a list of all new type errors:

npx tsc --noEmit --strict

Fix the highest-severity errors first. Use @ts-expect-error sparingly to suppress noise—this is a migration, not a codebase rewrite.

2. migrate target from ES5 → ES2025 (or latest supported).
Update your tsconfig.json:

{
  "compilerOptions": {
    "target": "es2025"
  }
}

Verify your bundler and deployment chain can handle the new output. Older clients will be unsupported without further transpilation.

3. update module resolution where necessary.
If you require CommonJS, set it explicitly:

{
  "compilerOptions": {
    "module": "commonjs"
  }
}

But review your dependencies—first-party packages and modern bundlers increasingly lean on ES modules. For libraries or tools that mandate ESNext, test their integration after the change.

4. automate migration and regression checks in CI.
Don’t trust local-only audits. In your CI pipeline:

# e.g. GitHub Actions step
- run: npx tsc --noEmit --strict
- run: npm run test

Fail builds for new type errors, but allow known, intentionally-suppressed issues (tracked via @ts-expect-error comments or exceptions).

5. incremental upgrades and deploys.
Don’t flip the version and settings in one PR. Instead:

• Upgrade to TS 6.0 with strict mode still disabled, running only compatibility tests.
• Enable strict mode after fixing blockers.
• Change target and module only after validating that local and cloud builds match.

Code freeze is a real risk on big codebases; incremental upgrades keep the pipeline flowing and the risk surface smaller.

What are the potential pitfalls and how to troubleshoot post-migration issues?

New type errors everywhere.
Strict mode surfaces latent issues. You’ll see waves of any-related or undefined value warnings:

function legacy(fn: any) {
  return fn.something?.()
  // TS now: Object is of type 'any'. Use of 'any' disables type checking.
}

Address them directly, or, if necessary, temporarily suppress using:

// @ts-expect-error
function legacy(fn: any) { ... }

Module bundling or import failures.
Moving to ESNext default can break import paths or runtime interop in Node.js. Babel, Webpack, or older code may not understand ES module syntax out of the box. Audit and update third-party tools and package.json accordingly.

Dropped legacy browser support.
Targeting es2025 kills Internet Explorer compatibility. If your users (or customers) depend on IE, establish a secondary transpilation pipeline (e.g., Babel post-process). TypeScript alone won’t do it after 6.0.

Cascading dependency upgrades.
If a dependency migrates to TS 6.0 and your pipeline doesn’t, your builds could fail due to downstream type errors or changed signature expectations.

Takeaway: expect problems, but manage them proactively and ship fixes in isolation. Suppress errors to buy time where needed, but don’t let them accumulate into technical debt.

Embrace strict migration—future-proof your stack

TypeScript 6.0 is the forced modernization step. Strict mode is the norm, ES5 is finally gone, and module resolution is finally pointed at the future. Migration isn’t negotiable—once dependencies flip, laggards will be broken by default.

Take the upgrade in stages. Flag and fix strict errors, explicitly set your targets, and document workarounds where needed. Every fix reduces the odds of a week lost to CI fire drills as dependencies move without you.

Stay ahead: the teams that embrace TypeScript 6.0's strict mode migration now will spend less time patching legacy traps—and more time building for whatever the Go-native 7.0 brings next.