feat: Add string insertion operators Append, Prepend, and Insert At

- Added Append operator to insert text at the end of the input
- Added Prepend operator to insert text at the start of the input
- Added Insert At operator to insert text at a given index within the input
- Added tests for all new operations.

src/core/config/Categories.json

@@ -349,7 +349,10 @@
             "Sleep",
             "File Tree",
             "Take nth bytes",
-            "Drop nth bytes"
+            "Drop nth bytes",
+            "Append",
+            "Prepend",
+            "Insert At"
         ]
     },
     {

src/core/operations/Append.mjs

@@ -0,0 +1,64 @@
+/**
+ * @author oliver-mitchell [oliver@polymerlabs.dev]
+ * @copyright Crown Copyright 2026
+ * @license Apache-2.0
+ */
+
+import Operation from "../Operation.mjs";
+import Utils from "../Utils.mjs";
+import { DELIM_OPTIONS } from "../lib/Delim.mjs";
+
+/**
+ * Append operation.
+ */
+class Append extends Operation {
+    /**
+     * Append constructor.
+     */
+    constructor() {
+        super();
+
+        this.name = "Append";
+        this.module = "Default";
+        this.description = "Add characters to the end of the input string, optionally with respect to a delimeter.";
+        this.inputType = "string";
+        this.outputType = "string";
+        this.args = [
+            {
+                "name": "Text",
+                "type": "binaryString",
+                "value": ""
+            },
+            {
+                "name": "Delimiter",
+                "type": "option",
+                "value": ["None", ...DELIM_OPTIONS]
+            },
+        ];
+    }
+
+    /**
+     * @param {string} input
+     * @param {Object[]} args
+     * @returns {string}
+     */
+    run(input, args) {
+        const [text, delim] = args;
+
+        if (delim === "None") {
+            return input + text;
+        }
+
+        const output = [],
+            delimChar = Utils.charRep(delim),
+            subs = input.split(Utils.charRep(delim));
+
+        for (let i = 0; i < subs.length; i++) {
+            output.push(subs[i] + text);
+        }
+
+        return output.join(delimChar);
+    }
+}
+
+export default Append;

src/core/operations/InsertAt.mjs

@@ -0,0 +1,88 @@
+/**
+ * @author oliver-mitchell [oliver@polymerlabs.dev]
+ * @copyright Crown Copyright 2026
+ * @license Apache-2.0
+ */
+
+import Operation from "../Operation.mjs";
+import Utils from "../Utils.mjs";
+import { DELIM_OPTIONS } from "../lib/Delim.mjs";
+
+/**
+ * Insert At operation.
+ */
+class InsertAt extends Operation {
+    /**
+     * InsertAt constructor.
+     */
+    constructor() {
+        super();
+
+        this.name = "Insert At";
+        this.module = "Default";
+        this.description = "Add characters to input string at a given index, optionally with respect to a delimeter.";
+        this.inputType = "string";
+        this.outputType = "string";
+        this.args = [
+            {
+                "name": "Text",
+                "type": "binaryString",
+                "value": ""
+            },
+            {
+                "name": "Index",
+                "type": "number",
+                "value": 0,
+                "min": 0
+            },
+            {
+                "name": "Delimiter",
+                "type": "option",
+                "value": ["None", ...DELIM_OPTIONS]
+            },
+        ];
+    }
+
+    /**
+     * @param {string} input
+     * @param {Object[]} args
+     * @returns {string}
+     */
+    run(input, args) {
+        const [text, index, delim] = args;
+
+        if (delim === "None") {
+            return InsertAt.performInsert(input, text, index);
+        }
+
+        const output = [],
+            delimChar = Utils.charRep(delim),
+            subs = input.split(Utils.charRep(delim));
+
+        for (let i = 0; i < subs.length; i++) {
+            output.push(InsertAt.performInsert(subs[i], text, index));
+        }
+
+        return output.join(delimChar);
+    }
+
+    /**
+     * @param {string} input
+     * @param {string} text
+     * @param {number} index
+     * @returns {string}
+     */
+    static performInsert(input, text, index) {
+        if (index >= input.length) {
+            return input + text;
+        }
+
+        if (index <= 0) {
+            return text + input;
+        }
+
+        return input.slice(0, index) + text + input.slice(index);
+    }
+}
+
+export default InsertAt;

src/core/operations/Prepend.mjs

@@ -0,0 +1,64 @@
+/**
+ * @author oliver-mitchell [oliver@polymerlabs.dev]
+ * @copyright Crown Copyright 2026
+ * @license Apache-2.0
+ */
+
+import Operation from "../Operation.mjs";
+import Utils from "../Utils.mjs";
+import { DELIM_OPTIONS } from "../lib/Delim.mjs";
+
+/**
+ * Prepend operation.
+ */
+class Prepend extends Operation {
+    /**
+     * Prepend constructor.
+     */
+    constructor() {
+        super();
+
+        this.name = "Prepend";
+        this.module = "Default";
+        this.description = "Add characters to the start of the input string, optionally with respect to a delimeter.";
+        this.inputType = "string";
+        this.outputType = "string";
+        this.args = [
+            {
+                "name": "Text",
+                "type": "binaryString",
+                "value": ""
+            },
+            {
+                "name": "Delimiter",
+                "type": "option",
+                "value": ["None", ...DELIM_OPTIONS]
+            },
+        ];
+    }
+
+    /**
+     * @param {string} input
+     * @param {Object[]} args
+     * @returns {string}
+     */
+    run(input, args) {
+        const [text, delim] = args;
+
+        if (delim === "None") {
+            return input + text;
+        }
+
+        const output = [],
+            delimChar = Utils.charRep(delim),
+            subs = input.split(Utils.charRep(delim));
+
+        for (let i = 0; i < subs.length; i++) {
+            output.push(text + subs[i]);
+        }
+
+        return output.join(delimChar);
+    }
+}
+
+export default Prepend;

tests/operations/tests/Append.mjs

@@ -0,0 +1,42 @@
+/**
+ * Media operation tests.
+ * @author oliver-mitchell [oliver@polymerlabs.dev]
+ * @copyright Crown Copyright 2026
+ * @license Apache-2.0
+ */
+import TestRegister from "../../lib/TestRegister.mjs";
+
+TestRegister.addTests([
+    {
+        name: "Append empty",
+        input: "test",
+        expectedOutput: "test",
+        recipeConfig: [
+            { op: "Append", args: ["", "None"] } // [text, delimiter]
+        ]
+    },
+    {
+        name: "Append empty input",
+        input: "",
+        expectedOutput: "test",
+        recipeConfig: [
+            { op: "Append", args: ["test", "None"] } // [text, delimiter]
+        ]
+    },
+    {
+        name: "Append with input and text",
+        input: "hello ",
+        expectedOutput: "hello world",
+        recipeConfig: [
+            { op: "Append", args: ["world", "None"] } // [text, delimiter]
+        ]
+    },
+    {
+        name: "Append with space delim",
+        input: "hello world",
+        expectedOutput: "hello: world:",
+        recipeConfig: [
+            { op: "Append", args: [":", "Space"] } // [text, delimiter]
+        ]
+    }
+]);

tests/operations/tests/InsertAt.mjs

@@ -0,0 +1,66 @@
+/**
+ * Media operation tests.
+ * @author oliver-mitchell [oliver@polymerlabs.dev]
+ * @copyright Crown Copyright 2026
+ * @license Apache-2.0
+ */
+import TestRegister from "../../lib/TestRegister.mjs";
+
+TestRegister.addTests([
+    {
+        name: "Insert At empty",
+        input: "test",
+        expectedOutput: "test",
+        recipeConfig: [
+            { op: "Insert At", args: ["", 0, "None"] } // [text, index, delimiter]
+        ]
+    },
+    {
+        name: "Insert At empty input",
+        input: "",
+        expectedOutput: "test",
+        recipeConfig: [
+            { op: "Insert At", args: ["test", 0, "None"] } // [text, index, delimiter]
+        ]
+    },
+    {
+        name: "Insert At with input and text",
+        input: "hello ",
+        expectedOutput: "hello world",
+        recipeConfig: [
+            { op: "Insert At", args: ["world", 6, "None"] } // [text, index, delimiter]
+        ]
+    },
+    {
+        name: "Insert At negative",
+        input: " world",
+        expectedOutput: "hello world",
+        recipeConfig: [
+            { op: "Insert At", args: ["hello", -5, "None"] } // [text, index, delimiter]
+        ]
+    },
+    {
+        name: "Insert At beyond string length",
+        input: "hello ",
+        expectedOutput: "hello world",
+        recipeConfig: [
+            { op: "Insert At", args: ["world", 100, "None"] } // [text, index, delimiter]
+        ]
+    },
+    {
+        name: "Insert At inside",
+        input: "heo",
+        expectedOutput: "hello",
+        recipeConfig: [
+            { op: "Insert At", args: ["ll", 2, "None"] } // [text, index, delimiter]
+        ]
+    },
+    {
+        name: "Insert At with space delim",
+        input: "hello world",
+        expectedOutput: "he:llo wo:rld",
+        recipeConfig: [
+            { op: "Insert At", args: [":", 2, "Space"] } // [text, index, delimiter]
+        ]
+    }
+]);

tests/operations/tests/Prepend.mjs

@@ -0,0 +1,42 @@
+/**
+ * Media operation tests.
+ * @author oliver-mitchell [oliver@polymerlabs.dev]
+ * @copyright Crown Copyright 2026
+ * @license Apache-2.0
+ */
+import TestRegister from "../../lib/TestRegister.mjs";
+
+TestRegister.addTests([
+    {
+        name: "Prepend empty",
+        input: "test",
+        expectedOutput: "test",
+        recipeConfig: [
+            { op: "Prepend", args: ["", "None"] } // [text, delimiter]
+        ]
+    },
+    {
+        name: "Prepend empty input",
+        input: "",
+        expectedOutput: "test",
+        recipeConfig: [
+            { op: "Prepend", args: ["test", "None"] } // [text, delimiter]
+        ]
+    },
+    {
+        name: "Prepend with input and text",
+        input: " world",
+        expectedOutput: "hello world",
+        recipeConfig: [
+            { op: "Prepend", args: ["hello", "None"] } // [text, delimiter]
+        ]
+    },
+    {
+        name: "Prepend with space delim",
+        input: "hello world",
+        expectedOutput: ":hello :world",
+        recipeConfig: [
+            { op: "Prepend", args: [":", "Space"] } // [text, delimiter]
+        ]
+    }
+]);