= Ignition
{disambiguate=JavaScript interpreter}
{c}
<Ignition (JavaScript interpreter)> is the register-based <interpreter> used within <V8>. It executes an internal representation of <JavaScript> computations rather than being a separate programming language with different source-level results.<Ignition in the V8 documentation>[Opening description, low-level register-based interpreter]{ref}
A small abstract <bytecode> example explains the role of such a representation. To compute a function returning `x + 2`, imagine instructions `load_arg r0; const r1, 2; add r2, r0, r1; return r2`. For argument `5`, the represented values are `r0 = 5`, `r1 = 2` and `r2 = 7`, so the result is `7`. An interpreter selects and performs the operation named by each instruction.
This instruction spelling is invented for explanation; it is not an Ignition disassembly or a promise about a particular V8 release. The example separates source syntax, an intermediate instruction representation and execution. A <register machine> supplies a related abstract way to reason about stored intermediate values and operations, without identifying its formal instruction set with a production engine's bytecode.
Actual <JavaScript> operations require more than an arithmetic mnemonic. The expression `x + 2` can produce numerical addition or string concatenation depending on the value of `x`. Supplying `"5"` yields `"52"` rather than `7`. Whatever internal instructions implement it must preserve the applicable <ECMAScript> behavior.<ECMAScript Language Specification by Ecma International>[Addition Operator, string concatenation and numeric addition]{ref}
Understanding <Ignition (JavaScript interpreter)> is therefore useful for studying execution and implementation costs, while the language specification remains the reference for observable program meaning.
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