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+# JavaScript ObjectSchema Package
+
+by [Nicholas C. Zakas](https://humanwhocodes.com)
+
+If you find this useful, please consider supporting my work with a [donation](https://humanwhocodes.com/donate).
+
+## Overview
+
+A JavaScript object merge/validation utility where you can define a different merge and validation strategy for each key. This is helpful when you need to validate complex data structures and then merge them in a way that is more complex than `Object.assign()`.
+
+## Installation
+
+You can install using either npm:
+
+```
+npm install @humanwhocodes/object-schema
+```
+
+Or Yarn:
+
+```
+yarn add @humanwhocodes/object-schema
+```
+
+## Usage
+
+Use CommonJS to get access to the `ObjectSchema` constructor:
+
+```js
+const { ObjectSchema } = require("@humanwhocodes/object-schema");
+
+const schema = new ObjectSchema({
+
+    // define a definition for the "downloads" key
+    downloads: {
+        required: true,
+        merge(value1, value2) {
+            return value1 + value2;
+        },
+        validate(value) {
+            if (typeof value !== "number") {
+                throw new Error("Expected downloads to be a number.");
+            }
+        }
+    },
+
+    // define a strategy for the "versions" key
+    version: {
+        required: true,
+        merge(value1, value2) {
+            return value1.concat(value2);
+        },
+        validate(value) {
+            if (!Array.isArray(value)) {
+                throw new Error("Expected versions to be an array.");
+            }
+        }
+    }
+});
+
+const record1 = {
+    downloads: 25,
+    versions: [
+        "v1.0.0",
+        "v1.1.0",
+        "v1.2.0"
+    ]
+};
+
+const record2 = {
+    downloads: 125,
+    versions: [
+        "v2.0.0",
+        "v2.1.0",
+        "v3.0.0"
+    ]
+};
+
+// make sure the records are valid
+schema.validate(record1);
+schema.validate(record2);
+
+// merge together (schema.merge() accepts any number of objects)
+const result = schema.merge(record1, record2);
+
+// result looks like this:
+
+const result = {
+    downloads: 75,
+    versions: [
+        "v1.0.0",
+        "v1.1.0",
+        "v1.2.0",
+        "v2.0.0",
+        "v2.1.0",
+        "v3.0.0"
+    ]
+};
+```
+
+## Tips and Tricks
+
+### Named merge strategies
+
+Instead of specifying a `merge()` method, you can specify one of the following strings to use a default merge strategy:
+
+* `"assign"` - use `Object.assign()` to merge the two values into one object.
+* `"overwrite"` - the second value always replaces the first.
+* `"replace"` - the second value replaces the first if the second is not `undefined`.
+
+For example:
+
+```js
+const schema = new ObjectSchema({
+    name: {
+        merge: "replace",
+        validate() {}
+    }
+});
+```
+
+### Named validation strategies
+
+Instead of specifying a `validate()` method, you can specify one of the following strings to use a default validation strategy:
+
+* `"array"` - value must be an array.
+* `"boolean"` - value must be a boolean.
+* `"number"` - value must be a number.
+* `"object"` - value must be an object.
+* `"object?"` - value must be an object or null.
+* `"string"` - value must be a string.
+* `"string!"` - value must be a non-empty string.
+
+For example:
+
+```js
+const schema = new ObjectSchema({
+    name: {
+        merge: "replace",
+        validate: "string"
+    }
+});
+```
+
+### Subschemas
+
+If you are defining a key that is, itself, an object, you can simplify the process by using a subschema. Instead of defining `merge()` and `validate()`, assign a `schema` key that contains a schema definition, like this:
+
+```js
+const schema = new ObjectSchema({
+    name: {
+        schema: {
+            first: {
+                merge: "replace",
+                validate: "string"
+            },
+            last: {
+                merge: "replace",
+                validate: "string"
+            }
+        }
+    }
+});
+
+schema.validate({
+    name: {
+        first: "n",
+        last: "z"
+    }
+});
+```
+
+### Remove Keys During Merge
+
+If the merge strategy for a key returns `undefined`, then the key will not appear in the final object. For example:
+
+```js
+const schema = new ObjectSchema({
+    date: {
+        merge() {
+            return undefined;
+        },
+        validate(value) {
+            Date.parse(value);  // throws an error when invalid
+        }
+    }
+});
+
+const object1 = { date: "5/5/2005" };
+const object2 = { date: "6/6/2006" };
+
+const result = schema.merge(object1, object2);
+
+console.log("date" in result);  // false
+```
+
+### Requiring Another Key Be Present
+
+If you'd like the presence of one key to require the presence of another key, you can use the `requires` property to specify an array of other properties that any key requires. For example:
+
+```js
+const schema = new ObjectSchema();
+
+const schema = new ObjectSchema({
+    date: {
+        merge() {
+            return undefined;
+        },
+        validate(value) {
+            Date.parse(value);  // throws an error when invalid
+        }
+    },
+    time: {
+        requires: ["date"],
+        merge(first, second) {
+            return second;
+        },
+        validate(value) {
+            // ...
+        }
+    }
+});
+
+// throws error: Key "time" requires keys "date"
+schema.validate({
+    time: "13:45"
+});
+```
+
+In this example, even though `date` is an optional key, it is required to be present whenever `time` is present.
+
+## License
+
+BSD 3-Clause