prefer-string-starts-ends-with.md

Expected Behavior: prefer-string-starts-ends-with Rule

This rule enforces the use of String.startsWith() and String.endsWith() over manual substring() comparisons for better readability and performance.

Examples

Basic String Checks

- if (str.substring(0, 3) === "abc") {
-   console.log('Starts with abc')
- }
+ if (str.startsWith("abc")) {
+   console.log('Starts with abc')
+ }

String Ends With Check

- if (str.substring(str.length - 3) === "xyz") {
-   console.log('Ends with xyz')
- }
+ if (str.endsWith("xyz")) {
+   console.log('Ends with xyz')
+ }

Function Returns

- function checkPrefix(str, prefix) {
-   return str.substring(0, prefix.length) === prefix
- }
+ function checkPrefix(str, prefix) {
+   return str.startsWith(prefix)
+ }

Function Returns Ends With

- function checkSuffix(str, suffix) {
-   return str.substring(str.length - suffix.length) === suffix
- }
+ function checkSuffix(str, suffix) {
+   return str.endsWith(suffix)
+ }

Variable Assignments

- const startsWithPrefix = str.substring(0, prefix.length) === prefix
+ const startsWithPrefix = str.startsWith(prefix)

Variable Assignment Ends With

- const endsWithSuffix = str.substring(str.length - suffix.length) === suffix
+ const endsWithSuffix = str.endsWith(suffix)

Method Calls

- if (getString().substring(0, 3) === "http") {
-   console.log('URL detected')
- }
+ if (getString().startsWith("http")) {
+   console.log('URL detected')
+ }

Method Call Ends With

- const isImage = filename.substring(filename.length - 4) === ".jpg"
+ const isImage = filename.endsWith(".jpg")

Property Access

- if (obj.name.substring(0, 2) === "Mr") {
-   console.log('Title found')
- }
+ if (obj.name.startsWith("Mr")) {
+   console.log('Title found')
+ }

Property Access Ends With

- const isEmail = user.email.substring(user.email.length - 4) === ".com"
+ const isEmail = user.email.endsWith(".com")

Rule Scope

This rule only applies to:

• ✅ str.substring(0, n) === "literal" patterns (startsWith)
• ✅ str.substring(str.length - n) === "literal" patterns (endsWith)
• ✅ Literal string comparisons only

This rule does NOT apply to:

• ❌ str.substring(0, prefix.length) === prefix (variable-based patterns)
• ❌ str.substring(str.length - suffix.length) === suffix (variable-based patterns)
• ❌ str.substring(1, 4) === "abc" (non-zero start index)
• ❌ Already using String.startsWith() or String.endsWith()
• ❌ Other string methods (indexOf, charAt, etc.)
• ❌ Different comparison operators (!==, >, <, etc.)

Covered Operations

This rule applies to the following operations:

• ✅ str.substring(0, 3) === "abc" → str.startsWith("abc")
• ✅ str.substring(str.length - 3) === "xyz" → str.endsWith("xyz")
• ✅ filename.substring(filename.length - 4) === ".jpg" → filename.endsWith(".jpg")

Auto-fix Behavior

The rule provides auto-fix suggestions that convert substring() comparisons to String.startsWith() and String.endsWith():

• str.substring(0, 3) === "abc" → str.startsWith("abc")
• str.substring(str.length - 3) === "xyz" → str.endsWith("xyz")
• filename.substring(filename.length - 4) === ".jpg" → filename.endsWith(".jpg")