Faster JavaScript Calls

JavaScript allows calling a function with a different number of arguments than the expected number of parameters, i.e., one can pass fewer or more arguments than the declared formal parameters. The former case is called under-application and the latter is called over-application.

In the under-application case, the remaining parameters get assigned the undefined value. In the over-application case, the remaining arguments can be accessed by using the rest parameter and the arguments property, or they are simply superfluous and they can be ignored. Many Web/NodeJS frameworks nowadays use this JS feature to accept optional parameters and create a more flexible API.

Until recently, V8 had a special machinery to deal with arguments size mismatch: the arguments adaptor frame. Unfortunately, argument adaption comes at a performance cost, but is commonly needed in modern front-end and middleware frameworks. It turns out that, with a clever trick, we can remove this extra frame, simplify the V8 codebase and get rid of almost the entire overhead.

We can calculate the performance impact of removing the arguments adaptor frame through a micro-benchmark.

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