JavaScript Computed Member Access

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Syntax

object[expression]

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Evaluation Mechanics

1. Base Evaluation: The object expression to the left of the brackets is evaluated to retrieve the base value.
2. Expression Evaluation: The expression inside the brackets is evaluated to retrieve the property name value.
3. Coercibility Check (RequireObjectCoercible): The engine checks if the base value is object-coercible. If the base evaluates to null or undefined, a TypeError is thrown. Because this check occurs after the bracket expression is evaluated but before key coercion, side effects from evaluating the expression itself will occur, but side effects from key coercion (such as invoking a custom toString() method on an object) will not execute.
4. Key Coercion (ToPropertyKey): The engine applies the internal ToPropertyKey abstract operation to the evaluated property name:
   • If the value is a Symbol, it is used directly as the property key.
   • For all other types, the value is coerced into a string using the internal ToString operation.
5. Reference Record Creation: The operator evaluates to a Reference Record containing the base value and the resolved property key.

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Type Coercion Behavior

const target = {};

// 1. Number coercion
target[1] = "A"; 
// Engine coercively converts 1 to "1". Equivalent to target["1"]

// 2. Boolean coercion
target[true] = "B"; 
// Engine coercively converts true to "true". Equivalent to target["true"]

// 3. Object coercion
const keyObj = {};
target[keyObj] = "C"; 
// keyObj.toString() is called. Equivalent to target["[object Object]"]

// 4. Symbol preservation
const sym = Symbol("id");
target[sym] = "D"; 
// Symbols bypass string coercion and remain unique identifiers.

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Identifier Constraints and Private Fields

const obj = {};

// Valid in bracket notation, syntax error in dot notation
obj["invalid-identifier"] = true;
obj["123starts_with_number"] = true;
obj[""] = true; // Empty string is a valid property key

class Example {
  #privateField = 10;

  test() {
    this.#privateField;    // Evaluates to 10 (Accesses private field)
    this["#privateField"]; // Evaluates to undefined (Accesses public property named "#privateField")
  }
}

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Array Indexing

const arr = ["X", "Y", "Z"];

arr[1]; 
// Evaluates to a Reference Record for base `arr` and property key "1".
// A subsequent [[Get]] operation on this Reference returns "Y".