GitHub Actions join() / toJSON Misuse on Non-Array Values

join() concatenates the elements of an array with a separator. Passing a scalar, or an array of objects, yields empty or surprising output because there is nothing simple to join.

What this error means

A join() expression returns an empty string or a row of separators with no values, usually when applied to a single value or to objects rather than an array of strings.

# matrix.os is a single string here, not an array - join produces nothing useful
run: echo "${{ join(matrix.os, ', ') }}"

Common causes

join() called on a scalar

join() expects an array. Passing a single string or number returns that value unchanged or empty, not a comma-separated list.

Array of objects, not strings

join() stringifies array elements. An array of objects produces empty or Object placeholders because objects have no useful string form.

How to fix it

Join an actual array of strings

# fromJSON produces a real array of strings
run: echo "${{ join(fromJSON('["linux","macos","windows"]'), ', ') }}"

Pick a scalar field before joining

1. If you have an array of objects, map to a scalar field first or build a string array.
2. Use toJSON to inspect a value when you are unsure of its shape.
3. For a single value, drop join() and reference the value directly.

How to prevent it

• Confirm a value is an array of scalars before calling join().
• Use toJSON to debug the actual shape of a context value.
• Build string arrays explicitly when you intend to join them.

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