Use explicit strict equality checks when testing for null or undefined values. Prefer x === undefined over typeof x === 'undefined' when the variable is known to exist. When checking for both null and undefined, use x == null (loose equality) as it covers both cases, or check each explicitly with x === null || x === undefined. Avoid using the || operator for fallback values when legitimate falsy values (like 0, false, or empty strings) are expected, as they will incorrectly trigger the fallback.

// Good - explicit undefined check
if (value !== undefined) {
  return value;
}

// Good - covers both null and undefined
if (value != null) {
  return value;
}

// Bad - unnecessary typeof when variable exists
if (typeof value !== 'undefined') {
  return value;
}

// Bad - loses legitimate falsy values
return node.style.marginTop || node.style.margin; // 0 becomes fallback value

// Good - explicit existence check
return 'marginTop' in node.style ? node.style.marginTop : node.style.margin;

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