How to return an object using the object name as a function parameter?

Using obj.bookName you are trying to access the bookName property of obj , which does not exist and therefore returns undefined.

You need to use obj[bookName] instead. So it will try to access the property whose name is equal to the value stored in the bookName variable.

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As luislhl said to access the property whose value comes in the variable passed as argument you will need to use obj[bookName] , but also remember to pass a string in the call of the book("livro1") function. If you call only using book(livro1) the interpreter will expect livro1 to be a previously declared variable.

var livro1 = "livro1"
book(livro1)

If you do not declare the variable or do not pass as a string the error Uncaught ReferenceError: livro1 is not defined will be fired.

You can use the eval() method to convert the bookName parameter, but call the function with 'book1' in quotation marks:

    function book(bookName) {
    	
    	var obj = {
    		livro1: {
    			quantidadePaginas: 300,
    			autor: "Jorge",
    			editora: "Atlas"
    		},
    		livro2: {
    			quantidadePaginas: 200,
    			autor: "Paulo",
    			editora: "Cia dos Livros"
    		},
    		livro3: {
    			quantidadePaginas: 150,
    			autor: "Pedro",
    			editora: "Bartolomeu"
    		}
    	};
    	if (!bookName) {
    		return obj;
    	};
    	return eval('obj.'+bookName);
    };
    
    console.log(book('livro1'));