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// We can check if an item or key exists by using a simple if statement.
// We will just provide the key to the if statement.
// If the item or key exists the if block will be executed.
if(dict.name){
console.log(dict.name);
}
if(dict[10]){
console.log(dict[10]);
}
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var person={"name":"Billy","age":20}
person.hasOwnProperty("name"); // true
person.hasOwnProperty("sex"); // false
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"key" in obj // true, regardless of the actual value
If you want to check if a key doesn't exist, remember to use parenthesis:
!("key" in obj) // true if "key" doesn't exist in object
!"key" in obj // ERROR! Equivalent to "false in obj"
Or, if you want to particularly test for properties of the object instance (and not inherited properties), use hasOwnProperty:
obj.hasOwnProperty("key") // true
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const person = {
firstName: 'John',
lastName: 'Doe',
age: 30,
city: 'New Work'
};
const keyToFind = 'age';
if (person.hasOwnProperty(keyToFind)) {
console.log('Key exists in the object!');
} else {
console.log('Key does not exist.');
}
// Output: "Key exists in the object!"
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!("key" in obj) // true if "key" doesn't exist in object
!"key" in obj // ERROR! Equivalent to "false in obj"
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var obj = { key: undefined };
obj["key"] !== undefined // false, but the key exists!
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5003
Checking for undefined-ness is not an accurate way of testing whether a key exists. What if the key exists but the value is actually undefined?
var obj = { key: undefined };
console.log(obj["key"] !== undefined); // false, but the key exists!
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var obj = { key: undefined };
obj["key"] !== undefined // false, but the key exists!