Nth Permutation Mathematics
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    2.0.7 • Public • Published


    Safe and fast serialization alternative to JSON.stringify.

    Gracefully handles circular structures instead of throwing.

    Provides a deterministic ("stable") version as well that will also gracefully handle circular structures. See the example below for further information.


    The same as JSON.stringify.

    stringify(value[, replacer[, space]])

    const safeStringify = require('fast-safe-stringify')
    const o = { a: 1 }
    o.o = o
    // '{"a":1,"o":"[Circular]"}'
    // TypeError: Converting circular structure to JSON
    function replacer(key, value) {
      console.log('Key:', JSON.stringify(key), 'Value:', JSON.stringify(value))
      // Remove the circular structure
      if (value === '[Circular]') {
      return value
    const serialized = safeStringify(o, replacer, 2)
    // Key: "" Value: {"a":1,"o":"[Circular]"}
    // Key: "a" Value: 1
    // Key: "o" Value: "[Circular]"
    // {
    //  "a": 1
    // }

    Using the deterministic version also works the same:

    const safeStringify = require('fast-safe-stringify')
    const o = { b: 1, a: 0 }
    o.o = o
    // '{"b":1,"a":0,"o":"[Circular]"}'
    // '{"a":0,"b":1,"o":"[Circular]"}'
    // TypeError: Converting circular structure to JSON

    A faster and side-effect free implementation is available in the [safe-stable-stringify][] module. However it is still considered experimental due to a new and more complex implementation.

    Differences to JSON.stringify

    In general the behavior is identical to JSON.stringify. The replacer and space options are also available.

    A few exceptions exist to JSON.stringify while using toJSON or replacer:

    Regular safe stringify

    • Manipulating a circular structure of the passed in value in a toJSON or the replacer is not possible! It is possible for any other value and property.

    • In case a circular structure is detected and the replacer is used it will receive the string [Circular] as the argument instead of the circular object itself.

    Deterministic ("stable") safe stringify

    • Manipulating the input object either in a toJSON or the replacer function will not have any effect on the output. The output entirely relies on the shape the input value had at the point passed to the stringify function!

    • In case a circular structure is detected and the replacer is used it will receive the string [Circular] as the argument instead of the circular object itself.

    A side effect free variation without these limitations can be found as well (safe-stable-stringify). It is also faster than the current implementation. It is still considered experimental due to a new and more complex implementation.


    Although not JSON, the Node.js util.inspect method can be used for similar purposes (e.g. logging) and also handles circular references.

    Here we compare fast-safe-stringify with some alternatives: (Lenovo T450s with a i7-5600U CPU using Node.js 8.9.4)

    fast-safe-stringify:   simple object x 1,121,497 ops/sec ±0.75% (97 runs sampled)
    fast-safe-stringify:   circular      x 560,126 ops/sec ±0.64% (96 runs sampled)
    fast-safe-stringify:   deep          x 32,472 ops/sec ±0.57% (95 runs sampled)
    fast-safe-stringify:   deep circular x 32,513 ops/sec ±0.80% (92 runs sampled)
    util.inspect:          simple object x 272,837 ops/sec ±1.48% (90 runs sampled)
    util.inspect:          circular      x 116,896 ops/sec ±1.19% (95 runs sampled)
    util.inspect:          deep          x 19,382 ops/sec ±0.66% (92 runs sampled)
    util.inspect:          deep circular x 18,717 ops/sec ±0.63% (96 runs sampled)
    json-stringify-safe:   simple object x 233,621 ops/sec ±0.97% (94 runs sampled)
    json-stringify-safe:   circular      x 110,409 ops/sec ±1.85% (95 runs sampled)
    json-stringify-safe:   deep          x 8,705 ops/sec ±0.87% (96 runs sampled)
    json-stringify-safe:   deep circular x 8,336 ops/sec ±2.20% (93 runs sampled)

    For stable stringify comparisons, see the performance benchmarks in the safe-stable-stringify readme.


    Whether fast-safe-stringify or alternatives are used: if the use case consists of deeply nested objects without circular references the following pattern will give best results. Shallow or one level nested objects on the other hand will slow down with it. It is entirely dependant on the use case.

    const stringify = require('fast-safe-stringify')
    function tryJSONStringify (obj) {
      try { return JSON.stringify(obj) } catch (_) {}
    const serializedString = tryJSONStringify(deep) || stringify(deep)


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