cache-loop-length.md

sfmc/ssjs-cache-loop-length

Require caching array .length in for loop conditions.

Type	suggestion
Default severity	warn in recommended; error in strict
Fixable	Auto-fix (eslint --fix)

Why This Rule Exists

SFMC's SSJS engine (JINT) re-evaluates the loop test expression on every iteration. A condition like i < arr.length therefore calls the .length getter on every pass through the loop. For large arrays this is a measurable performance cost. The recommended pattern (per Mateusz Dąbrowski's SSJS style guide) is to cache the length in a variable in the loop initialiser:

for (var i = 0, _len = arr.length; i < _len; i++) { … }

Settings

Setting	Values	Default
severity	"error" | "warn" | "off"	"warn"

This rule has no configuration options.

Examples

Not allowed:

for (var i = 0; i < items.length; i++) {
    process(items[i]);
}

Allowed:

for (var i = 0, _len = items.length; i < _len; i++) {
    process(items[i]);
}

Fix

This rule provides an auto-fix. Applied by:

• eslint --fix on the command line
• Fix this issue / Fix all auto-fixable problems in VS Code (requires the ESLint extension)
• On save via editor.codeActionsOnSave: { "source.fixAll.eslint": "explicit" }

What the auto-fix does: appends , _len = arr.length to the last declarator in the for loop initialiser and replaces arr.length in the test condition with _len. The fix is only applied when the loop init is a var declaration (the typical case). Loops with other init forms (for example for (i = 0; i < arr.length; i++)) are still reported but not auto-fixed.

When to Disable

// eslint.config.js
rules: { 'sfmc/ssjs-cache-loop-length': 'off' }