如何在JavaScript中处理未定义的值

let variableName = null;

function exampleFunction(param1, param2 = 'defaultValue') {
  // ...
}

if ('propertyName' in object) {
  // ...
}

if (object.hasOwnProperty('propertyName')) {
  // ...
}

if (typeof variableName === 'undefined') {
  // ...
}

const variableName = someVariable || 'defaultValue';

const variableName = someVariable || null;
const variableName = someVariable || undefined;

'use strict';