WEBASSEMBLY / CORE CONCEPTS
Locals, globals, stack values, and mutability
Learn how locals, globals, stack values, and mutability works in WebAssembly, why the underlying model matters, and how to apply it in a small program without hiding important trade-offs.
What you will learn
- Explain locals in WebAssembly using the correct mental model
- Trace a focused WebAssembly example and predict its result before execution
- Recognize a boundary case involving globals and handle it deliberately
Understanding Locals, globals, stack values, and mutability
Locals, globals, stack values, and mutability belongs to the practical core of WebAssembly. Start by identifying the values or state involved and the rule that connects the input to the result.
Trace the example one operation at a time. Keep locals visible in the code rather than hiding it behind an abstraction before the behavior is understood.
Test a normal case and a boundary case. The difference between the prediction and the observed result is the most useful signal for deciding what to review next.
Locals, globals, stack values, and mutability is a defining part of practical WebAssembly work. Start by identifying the data or state involved, then trace the operation that changes or interprets it. Pay attention to the rules WebAssembly applies at this boundary, because those rules explain both the useful behavior and the common failure modes. This lesson keeps the example deliberately small, then connects it to numeric types, references, tables, and memory so the ideas form a coherent progression rather than a list of isolated syntax facts.
Worked examples
Locals, globals, stack values, and mutability example
A focused WebAssembly example for locals.
;; Textual WebAssembly example
(module
(func (export "answer") (result i32)
i32.const 42))
;; Lesson 3: locals. Change one value and predict the result before running it.Example explained
Line 1Identify where locals appears in the WebAssembly example and name the data it operates on.
Line 2Trace the relevant WebAssembly rule one operation at a time, recording any state, type, or control-flow change.
Line 3Change one input or boundary condition, predict the result, and compare that prediction with the documented outcome.
Important notes
Keep the first locals example small enough to trace completely.
Use the normal WebAssembly toolchain or browser workspace to compare the actual result with your prediction.
Common mistakes
Treating locals as punctuation to memorize instead of a WebAssembly behavior to reason about.
Ignoring globals until it appears in production data or a larger program.
Try it yourself
Change, predict, then run
Create a small WebAssembly example that demonstrates locals. Add a normal case and a boundary case, write down the expected result for each, then explain which WebAssembly rule produces that result. Lesson 3 should remain small enough to trace without guessing.
Open WebAssembly workspaceCheck your understanding
What is the best first step when working with locals?
- Identify the data and predict the result
- Add more abstraction immediately
- Ignore boundary cases
- Memorize punctuation only
Show answer
A clear input, operation, and predicted result create a testable mental model.