How can a field be added to an object depending on a specified condition?

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To add a field to an object depending on a specified condition, you can use a conditional statement in your programming language. Here is an example using JavaScript:

var person = {name: "John", age: 25};
if (person.age >= 18) {
    person.isAdult = true;
} else {
    person.isAdult = false;
}
console.log(person); // prints {name: "John", age: 25, isAdult: true}

In this example, we check if the person's age is greater than or equal to 18. If it is, we add a new field called "isAdult" with the value of true. If it is not, we add the "isAdult" field with the value of false.