JavaScriptの'this'バインディング：コンテキスト切り替えとアロー関数の挙動をマスターする

            function globalFunction() {
  console.log(this === window); // true (without strict mode)
  console.log(this); // window object (without strict mode)
}

globalFunction();

            

              
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            function globalFunction() {
  console.log(this === global); // true (without strict mode)
  console.log(this); // global object (without strict mode)
}

globalFunction();

            

              
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            "use strict";
function globalFunction() {
  console.log(this === undefined); // true
  console.log(this); // undefined
}

globalFunction();

            

              
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            const myObject = {
  name: "Example Object",
  greet: function() {
    console.log("Hello, my name is " + this.name);
  }
};

myObject.greet(); // Output: Hello, my name is Example Object

            

              
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            function.call(thisArg, arg1, arg2, ...)

            

              
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            const person = {
  name: "Alice",
  greet: function(greeting) {
    console.log(greeting + ", my name is " + this.name);
  }
};

const anotherPerson = {
  name: "Bob"
};

person.greet.call(anotherPerson, "Hello"); // Output: Hello, my name is Bob

            

              
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            function.apply(thisArg, [argsArray])

            

              
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            const person = {
  name: "Alice",
  greet: function(greeting, punctuation) {
    console.log(greeting + ", my name is " + this.name + punctuation);
  }
};

const anotherPerson = {
  name: "Bob"
};

person.greet.apply(anotherPerson, ["Hello", "!"]); // Output: Hello, my name is Bob!

            

              
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            function.bind(thisArg, arg1, arg2, ...)

            

              
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            const person = {
  name: "Alice",
  greet: function(greeting) {
    console.log(greeting + ", my name is " + this.name);
  }
};

const anotherPerson = {
  name: "Bob"
};

const greetBob = person.greet.bind(anotherPerson, "Hello");
greetBob(); // Output: Hello, my name is Bob

            

              
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            function Person(name) {
  this.name = name;
  this.greet = function() {
    console.log("Hello, my name is " + this.name);
  };
}

const alice = new Person("Alice");
alice.greet(); // Output: Hello, my name is Alice

            

              
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            const myObject = {
  name: "Example Object",
  greet: function() {
    setTimeout(() => {
      console.log("Hello, my name is " + this.name); // this refers to myObject
    }, 1000);
  }
};

myObject.greet(); // Output (after 1 second): Hello, my name is Example Object

            

              
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            const myObject = {
  name: "Example Object",
  greet: function() {
    const self = this; // Capture 'this'
    setTimeout(function() {
      console.log("Hello, my name is " + self.name); // Need to use 'self'
    }, 1000);
  }
};

myObject.greet();

            

              
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            <button id="myButton">Click Me</button>

<script>
  const button = document.getElementById("myButton");
  button.addEventListener("click", function() {
    console.log(this === button); // true
    this.textContent = "Clicked!"; // Change button text
  });
</script>

            

              
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            class Rectangle {
  constructor(width, height) {
    this.width = width;
    this.height = height;
  }

  getArea() {
    return this.width * this.height;
  }
}

const myRectangle = new Rectangle(10, 5);
console.log(myRectangle.getArea()); // Output: 50

            

              
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            function fetchData(callback) {
  // Simulate an asynchronous operation
  setTimeout(() => {
    const data = { message: "Data fetched successfully" };
    callback(data);
  }, 1000);
}

const myObject = {
  name: "My Object",
  processData: function() {
    fetchData((data) => {
      console.log(this.name + ": " + data.message); // 'this' refers to myObject
    });
  }
};

myObject.processData(); // Output (after 1 second): My Object: Data fetched successfully

            

              
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            function createCounter() {
  let count = 0;
  return {
    increment: function() {
      count++;
      console.log(count);
    },
    getCount: function() {
      return count;
    }
  };
}

const counter = createCounter();
counter.increment(); // Output: 1
counter.increment(); // Output: 2
console.log(counter.getCount()); // Output: 2

            

              
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