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Episode 2: How JavaScript Code is executed?

Execution Context and Phases​

  • JavaScript code is executed in execution contexts.
  • Execution contexts have two phases: memory creation and code execution.

Memory Creation Phase​

  • In the memory creation phase, memory is allocated to variables and functions.
  • Variables are assigned the value of undefined.
  • Functions are stored in memory as they are.

Code Execution Phase​

  • In the code execution phase, the code is executed line by line.
  • Variables created in Memory Creation Phase start assigning value as code executed.
  • Functions are invoked by creating a new execution context (Global Execution Context).
  • The execution context has its own memory and code components.
  • Code inside the function is executed.
  • return statements return control to the invoking context.
  • Return values are stored in memory if stored in variable.
  • The execution context is deleted after the function finishes.

Execution Context Working

Call Stack​

  • The call stack manages the execution contexts.
  • It maintains the order of execution of execution contexts.
  • Each new execution context is pushed onto the stack.
  • The topmost execution context is the one currently being executed.
  • The bottom execution context is the Global Execution Context and others are Function EC.
  • When a function finishes, its execution context is popped from the stack.
  • Eventually GEC is also popped from the stack, Execution of program completed.

Call Stack

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How JS is executed & Call Stack Youtube Link