Node.js util.format() Method
The util.format() (Added in v0.5.3) method is an inbuilt application programming interface of the util module which is like printf format string and returns a formatted string using the first argument. The formatted string contains zero or more format specifiers in which the corresponding argument value is converted and replaced. It is used as a debugging tool is a synchronous method hence, it can have a significant performance overhead that could block the event loop. It is recommended not to use this function in a hot code path.
Syntax:
util.format(format[, ...args])
Parameters: This method accept two parameters as mentioned above and described below:
format: It consists of specifiers of <string> type, which is like printf format string.
args: It is the <string> type list of arguments.
Supported specifiers are:
- %%: It replaces specifier with a single percent sign (‘%%’/‘%’) and doesn’t consume any argument even if it provided.
- %s (String): It converts all values according to given format except Object, BigInt, and -0. Objects that have no user-defined toString function are inspected using util.inspect() and BigInt values are represented with n.
- %c(CSS): If any CSS is passed, it will be skipped. Usually, this specifier is ignored.
- %d (Number): It converts all values according to the given format except Symbol and BigInt.
- %i (parseInt()): It parses a <string> and returns an integer, it is used for all values except BigInt and Symbol.
- %f (parseFloat()): It parses a string and returns a floating-point number, it is used for all values except Symbols.
- %j (JSON): No complicated parsing or translations, if there are circular references in the argument then it is replaced with the string ‘[Circular]’.
- %o(Object): It is a string representation of an object with generic JavaScript object formatting. Similar to util.inspect(). It shows the full object along with non-enumerable properties and proxies.
- %O(Object): It is similar to ‘%o’ but without options, it does not include non-enumerable properties and proxies.
Return Value: It returns the formatted string of <string> type.
Example 1:
Filename: index.js
javascript
// Node.js to demonstrate the// util.format() method // Import the util module const util = require('util'); function fun1() { var val1 = util.format('%s:%s:%s', 'abc'); // Returns: 'foo:%s' var val2 = util.format('%s:%s', 'abc', 'def', 'ghi', 'jkl'); // Returns: 'foo:bar baz' var val3 = util.format(10, 20, 30); // Returns: '1 2 3' var val4 = util.format('%% : %s : %d'); // Returns: '%% %s' var val5 = util.format('%% : %s', 567); // Returns: '% : 567' console.log(val1, '\n', val2, '\n', val3, '\n', val4, '\n', val5);} // Function callfun1(); |
Run index.js file using the following command:
node index.js
Output:
abc: :abc:def ghi jkl 10 20 30 %% : %s : %d % : 567
Example 2:
Filename: index.js
javascript
// Node.js program to demonstrate// the util.format() method // Import the util module const util = require('util'); // Passing multiple values and// -0 on string specifierconsole.log("1.>", util.format( '%%: %s', 'abc', 'def', -0)); // Passing multiple values console.log("2.>", util.format( '%%', 'abc', 'def', 'ghi')); // Passing bigInt to string specifierconsole.log("3.>", util.format('%s', 'abc', 94321321321223372036854775807)); // Creating and passing Object along // with null prototype and a variableconsole.log("4.>", util.format('%s', 'abc', Object.create(null, { [Symbol.toStringTag]: { value: 'def' } }))); // Passing string to Number specifierconsole.log("5.>", util.format('%d', 'abc', 94303685)); // Passing Symbol and Number to// parseInt specifierconsole.log("6.>", util.format( '%i', '2020 year 2021, ', 'He was 40,' , '10.33, ', '10, ', 10)); // Passing string and Numbers// to parseFloat specifierconsole.log("7.>", util.format('%f', '94321321321.564000 year 6546', 'abc', 943036854775807)); // Passing JSON string and Number// to JSON specifierconsole.log("8.>", util.format('%j', '{ "name":"John", "age":31, "city":"New York" }', 'abc', 943036854775807)); // Passing class, string, and Number// to object specifierconsole.log("9.>", util.format('%o', class Bar { }, 'abc', 943036854775807)); // Passing class, string, and Number// to Object specifierconsole.log("10.>", util.format('%o:%d', class Foo { get [Symbol.toStringTag]() { return 'abc'; } }, 'abc', 943036854775807)); // Random classclass randomClass { } // Inspecting random classconsole.log("11.>", util.inspect(new randomClass())); |
Run index.js file using the following command:
node index.js
Output:
1.> %: abc def -0
2.> % abc def ghi
3.> abc 9.432132132122338e+28
4.> abc [Object: null prototype] [def] {}
5.> NaN 94303685
6.> 2020 He was 40, 10.33, 10, 10
7.> 94321321321.564 abc 943036854775807
8.> "{ \"name\":\"John\", \"age\":31,
\"city\":\"New York\" }" abc 943036854775807
9.> <ref *1> [Function: Bar] {
[length]: 0,
[prototype]: Bar { [constructor]: [Circular *1] },
[name]: 'Bar'
} abc 943036854775807
10.> <ref *1> [Function: Foo] {
[length]: 0,
[prototype]: Foo {
[constructor]: [Circular *1],
[Symbol(Symbol.toStringTag)]: [Getter]
},
[name]: 'Foo'
}:NaN 943036854775807
11.> randomClass {}Conditions:
- If no corresponding argument is passed to the specifier then it is not replaced.
- If multiple arguments are passed than the number of specifiers, then extra arguments will be concatenated to the returned string.
- If ‘values’ do not belong to format string and their type is not a string then they are formatted using util.inspect() method.
- If the first argument doesn’t have a valid format specifier, then util.format() returns concatenated arguments.
Reference: https://nodejs.org/api/util.html#util_util_format_format_args
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