Quick answer: Parameters are zero-indexed: Function.Param(0) for the first. Ensure the function signature declares enough parameters and the types match (Number vs String). For object references, pass UID as a number and reacquire via Pick by UID.

Here is how to fix Construct 3 function parameters not passed correctly. You call a function with three parameters: ApplyDamage(enemy.UID, 25, "fire"). Inside the function, Function.Param(1) returns 0 instead of 25. Or Function.Param(2) returns an empty string instead of “fire.” Or the first parameter works but later ones are missing. Construct 3’s Function object is straightforward but strict about indexing and types.

The Symptom

A Function object call passes parameters but the callee sees wrong values:

What Causes This

Zero-based indexing. Function.Param(0) is the first parameter, not the second. Developers used to 1-based UIs sometimes write Function.Param(1) expecting the first param and get the second (or nothing if only one was passed).

Parameter count mismatch. The function’s declaration in On Function event specifies parameter count. If the caller passes fewer parameters, the missing ones default to 0 (for Number) or empty string (for String). If the caller passes more, extras are silently dropped.

Type mismatch. Parameters have declared types (Number or String). If you try to use a String parameter in a math expression, Construct coerces to 0. Same for passing a Number where the function expects String.

SOL (Scope of Logic) differences. Functions start with a fresh SOL — the callee does not inherit the caller’s picked instances. If your caller had Enemy picked to a specific instance and the function expects to operate on that Enemy, the function sees all Enemies, not the picked one. Pass the UID and pick by UID in the function.

Async issues. Async functions run asynchronously via promises. Wait for the result with Wait for Previous Actions to Complete or handle the promise. Premature reads get stale values.

The Fix

Step 1: Audit parameter indexes. For a three-parameter function:

On Function "ApplyDamage":
  Param(0)  -> enemy_uid    // first parameter
  Param(1)  -> damage       // second parameter
  Param(2)  -> element_type // third parameter

Always zero-based. Match your mental model to this convention.

Step 2: Declare parameters in Function signature. In the On Function event’s signature (visible in the Functions sidebar), declare each parameter with name and type. A declared signature:

Named parameters are self-documenting and reduce errors:

On Function "ApplyDamage" (UID Number, Amount Number, Element String):
  Enemy: Pick by UID == Function.UID
  Enemy: Subtract Function.Amount from Health

Step 3: Pass object UIDs, reacquire in callee.

// Caller: pass UID to preserve object reference
Enemy on collision with Bullet:
  Call Function "ApplyDamage" (Enemy.UID, 25, "fire")

// Callee: reacquire using UID
On Function "ApplyDamage":
  Enemy: Pick by UID == Function.UID
  Enemy: Subtract Function.Amount from Health

This pattern keeps the callee scoped to the specific enemy instead of iterating all enemies.

Step 4: Match types deliberately. For Number types, use Function.Param for math. For String, use Function.Param in text operations or comparisons. Mixed usage produces implicit conversion that may not be what you want.

// Correct: Number parameter in arithmetic
Text: Set text to "Damage: " & Function.Param(1)

// Wrong: String parameter in math
// Enemy.Health = Enemy.Health - Function.Param(2) // subtracts 0 if param is string

Debugging Parameter Values

Log parameters at function entry to verify:

On Function "ApplyDamage":
  Browser: Log "ApplyDamage called: uid=" & Function.Param(0)
    & ", dmg=" & Function.Param(1)
    & ", elem=" & Function.Param(2)

Open browser console during preview. The log shows exact values received. Mismatch from what you expected tells you where the bug is — caller or callee.

Script-Based Alternative

For complex parameter handling, JavaScript functions are cleaner:

// In a Script action
function applyDamage(uid, damage, element) {
    const enemy = runtime.getInstanceByUid(uid);
    if (enemy) {
        enemy.instVars.Health -= damage;
        enemy.instVars.LastElement = element;
    }
}

No indexing, no type coercion issues, same performance. For large projects with many parameters, Script is often cleaner.

Understanding the issue

This bug class falls into a pattern that's worth understanding beyond the specific case. In Construct 3, the underlying behavior is shaped by how the engine layers its abstractions - the public API you call, the runtime systems that respond, and the platform-specific implementations underneath. A bug at any layer can produce symptoms that look like they originate at a different layer. Triaging effectively means recognizing which layer the symptom belongs to, even when the gameplay code is what's visible.

The specific bug described above is the kind that surfaces during integration rather than unit testing. It depends on a combination of factors: the asset configuration, the runtime state, the platform's specific behavior. In isolation, each piece looks correct; in combination, the bug emerges. This is why thorough integration testing - playing the actual game in realistic conditions - catches things that automated tests miss.

Why this happens

This bug class disproportionately affects late-stage development. The work to surface it is interactive testing in realistic conditions, which only really happens after the gameplay is in place and assets are populated. Catching it early requires deliberate testing of conditions that look unimportant.

At the engine level, the behavior comes from a deliberate design decision in Construct 3. The engine team chose a particular trade-off - usually performance versus convenience, or generality versus specificity - and that trade-off has consequences when you push against it. Understanding the trade-off is what turns 'this bug is mysterious' into 'this bug is the expected consequence of this design'.

Verifying the fix

After applying the fix, the verification step has three parts: confirm the original repro is resolved, confirm no obvious regressions in adjacent functionality, and (for shipping titles) deploy to a small player cohort first and watch the crash and report rates. Each step catches something the others miss.

Reproducibility is the prerequisite for verification. If you can't reliably reproduce the bug pre-fix, you can't reliably verify it post-fix. Spend time getting a clean reproduction before you write any fix code. The fix is fast once you understand the reproduction; the reproduction is the slow part.

Variations to watch for

There's almost always a less obvious case where the same problem applies. The reported case is the one a player hit; the related cases hide because they're rarer or affect fewer players. After fixing the reported case, search the codebase for the pattern - one fix often unlocks several.

Adjacent bugs often share a root cause. After fixing the case you've found, spend an hour searching the codebase for similar patterns. What's the same call with different arguments? The same data flow with a different entity type? The same lifecycle issue in a sibling system? Each match is a candidate for the same fix, or a related fix that prevents future bugs of the same class.

In production

In shipping builds, this issue may interact with other production-only behavior. Stripping, encryption, asset bundling, and platform-specific code paths can each modify the symptoms. When players report a related issue, capture build SHA, platform, and any feature flags - those three fields cover most of the production-only variations.

When triaging a similar issue in production, prioritize gathering data over hypothesizing causes. A player report describes a symptom; what you need is a build SHA, a session timestamp, and ideally a screen recording or session replay. With those, the bug becomes tractable. Without them, you're guessing at hypothetical reproductions that may not match what the player actually hit.

Performance considerations

Performance implications matter when this bug class scales with player count or asset count. A bug that fires once per session is annoying; a bug that fires once per frame compounds. After fixing, profile the affected code path under realistic load. The fix that's correct for one entity may be too slow for ten thousand.

Diagnostic approach

Diagnosing this class of bug benefits from a structured approach: confirm the symptom, isolate the variables, hypothesize the cause, and verify the hypothesis before writing fix code. Skipping the isolation step is the most common mistake; without it, fixes often address symptoms while the underlying cause continues to produce other variations.

For Construct 3-specific diagnostics, the editor's profiler is the canonical starting point. Capture a representative frame with the symptom present; compare against a frame without the symptom; the diff often points directly at the cause. If the symptom is non-deterministic, capture multiple frames and look for the pattern - the cause is usually a state transition or a specific input value rather than a continuous effect.

Tooling and ecosystem

Modern engine versions ship better tooling for this kind of issue than older versions. If you're on an older release, the diagnostic step may take significantly longer because the tools you'd want don't exist yet. Sometimes the right answer is upgrading rather than fighting through limited tooling.

Within Construct 3, the relevant diagnostic surfaces include the standard frame debugger, memory profiler, and engine-specific debug overlays. Each one shows a different facet of what's happening. The frame debugger reveals draw call ordering and state transitions; the memory profiler shows allocation patterns; the debug overlay reveals per-system state. Bugs that resist one tool usually surrender to another - the trick is knowing which tool to reach for first.

Edge cases and pitfalls

Platform-specific edge cases are worth enumerating explicitly. iOS handles backgrounding differently than Android; Windows handles focus changes differently than macOS. A fix that works on the development platform may not work on every target. Test on each shipping platform deliberately.

When writing a regression test for this fix, focus on the boundary conditions that surfaced the original bug. Tests that exercise the happy path catch obvious regressions; tests that exercise the boundary catch the subtler regressions that look like new bugs but are really the original returning. The latter are the tests that earn their keep over the long life of the project.

Team communication

When this bug class affects multiple teams (often the case for cross-system issues), early communication prevents duplicate work. The team that owns the symptom may not own the cause. A 15-minute conversation at the start of triage often saves hours of independent investigation.

If this fix touches a system several engineers work in, a short writeup in the team's engineering channel helps. Not a full design doc - a paragraph explaining what was wrong, what's fixed, and what to watch for. Future engineers encountering similar symptoms will search for the fix; making it findable is a small investment that pays back later.

“Function parameters are zero-indexed, typed, and lose SOL context. Named parameters + UID passing cover the most common bugs.”

Related Issues

For collision issues, see Construct 3 Collision Not Detecting. For performance, Construct 3 Performance Tips covers related patterns.

Named parameters. Pass UID and Pick by UID. Zero-indexed. Three rules cover 90% of bugs.