Migrate to Strict TypeScript Incrementally

Intermediate15 minTrending

Enable strict mode in an existing TypeScript project one flag at a time to avoid a massive migration and catch type errors gradually.

Prerequisites

-Existing TypeScript project
-Working build pipeline

Steps

Check current strictness level

Review your tsconfig.json to see which strict flags are already enabled.

$ cat tsconfig.json | grep -A 20 'compilerOptions'

Enable strict flags one at a time

Instead of enabling strict: true all at once, enable individual flags and fix errors incrementally.

$ npx tsc --noEmit --noImplicitAny 2>&1 | head -30

Recommended order: noImplicitAny, strictNullChecks, strictFunctionTypes, strictBindCallApply, noImplicitThis, alwaysStrict.

Count errors for each strict flag

Measure the scope of each flag before committing to fix it.

$ echo "noImplicitAny: $(npx tsc --noEmit --noImplicitAny 2>&1 | grep -c 'error TS')" && echo "strictNullChecks: $(npx tsc --noEmit --strictNullChecks 2>&1 | grep -c 'error TS')"

Enable strictNullChecks and fix null errors

This is typically the most impactful flag. Fix undefined access patterns with optional chaining and nullish coalescing.

Use optional chaining (?.) and nullish coalescing (??) instead of non-null assertions (!) wherever possible.

Non-null assertions (!) silence the compiler but do not fix the underlying issue. Prefer proper null checks.

Enable full strict mode

Once all individual flags pass, enable strict: true to lock in all current and future strict checks.

$ npx json -I -f tsconfig.json -e 'this.compilerOptions.strict = true' && npx tsc --noEmit

After enabling strict: true, you can remove the individual flags since strict enables all of them.

Full Script

full-script.sh

#!/bin/bash
# Incremental Strict TypeScript Migration
set -euo pipefail

echo "=== Current TypeScript Errors ==="
CURRENT=$(npx tsc --noEmit 2>&1 | grep -c 'error TS' || true)
echo "Current errors: $CURRENT"

echo ""
echo "=== Error Count Per Strict Flag ==="
for flag in noImplicitAny strictNullChecks strictFunctionTypes strictBindCallApply noImplicitThis alwaysStrict; do
  COUNT=$(npx tsc --noEmit --$flag 2>&1 | grep -c 'error TS' || true)
  echo "$flag: $COUNT errors"
done

echo ""
echo "=== Recommended Migration Order ==="
echo "1. alwaysStrict        (usually zero errors)"
echo "2. noImplicitThis      (rare in modern React/Node)"
echo "3. strictBindCallApply (low impact)"
echo "4. strictFunctionTypes (moderate impact)"
echo "5. noImplicitAny       (high impact, most common)"
echo "6. strictNullChecks    (highest impact, most valuable)"

FAQ

Discussion

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#!/bin/bash # Incremental Strict TypeScript Migration set -euo pipefail echo "=== Current TypeScript Errors ===" CURRENT=$(npx tsc --noEmit 2>&1 | grep -c 'error TS' || true) echo "Current errors: $CURRENT" echo "" echo "=== Error Count Per Strict Flag ===" for flag in noImplicitAny strictNullChecks strictFunctionTypes strictBindCallApply noImplicitThis alwaysStrict; do COUNT=$(npx tsc --noEmit --$flag 2>&1 | grep -c 'error TS' || true) echo "$flag: $COUNT errors" done echo "" echo "=== Recommended Migration Order ===" echo "1. alwaysStrict (usually zero errors)" echo "2. noImplicitThis (rare in modern React/Node)" echo "3. strictBindCallApply (low impact)" echo "4. strictFunctionTypes (moderate impact)" echo "5. noImplicitAny (high impact, most common)" echo "6. strictNullChecks (highest impact, most valuable)"