Implement concat

Author: MarkDuckworth

packages/firestore/src/api_pipelines.ts

@@ -162,6 +162,7 @@ export {
   stringReverse,
   length as len,
   abs,
+  concat,
   Expression,
   AliasedExpression,
   Field,
@@ -174,7 +175,7 @@ export {
 export type {
   ExpressionType,
   AggregateWithAlias,
-  Selectable,
+  Selectable
 } from './lite-api/expressions';
 
 export { _internalPipelineToExecutePipelineRequestProto } from './remote/internal_serializer';

packages/firestore/src/lite-api/expressions.ts

@@ -1046,6 +1046,27 @@ export abstract class Expression implements ProtoValueSerializable, UserData {
     );
   }
 
+  /**
+   * Creates an expression that concatenates expression results together.
+   *
+   * ```typescript
+   * // Combine the 'firstName', ' ', and 'lastName' fields into a single value.
+   * field("firstName").concat(constant(" "), field("lastName"));
+   * ```
+   *
+   * @param second The additional expression or literal to concatenate.
+   * @param others Optional additional expressions or literals to concatenate.
+   * @return A new `Expr` representing the concatenated value.
+   */
+  concat(
+    second: Expression | unknown,
+    ...others: Array<Expression | unknown>
+  ): FunctionExpression {
+    const elements = [second, ...others];
+    const exprs = elements.map(valueToDefaultExpr);
+    return new FunctionExpression('concat', [this, ...exprs], 'concat');
+  }
+
   /**
    * Creates an expression that reverses this string expression.
    *
@@ -7142,6 +7163,56 @@ export function stringReverse(expr: Expression | string): FunctionExpression {
   return fieldOrExpression(expr).stringReverse();
 }
 
+/**
+ * Creates an expression that concatenates strings, arrays, or blobs. Types cannot be mixed.
+ *
+ * ```typescript
+ * // Concatenate the 'firstName' and 'lastName' fields with a space in between.
+ * concat(field("firstName"), " ", field("lastName"))
+ * ```
+ *
+ * @param first The first expressions to concatenate.
+ * @param second The second literal or expression to concatenate.
+ * @param others Additional literals or expressions to concatenate.
+ * @return A new `Expression` representing the concatenation.
+ */
+export function concat(
+  first: Expression,
+  second: Expression | unknown,
+  ...others: Array<Expression | unknown>
+): FunctionExpression;
+
+/**
+ * Creates an expression that concatenates strings, arrays, or blobs. Types cannot be mixed.
+ *
+ * ```typescript
+ * // Concatenate a field with a literal string.
+ * concat(field("firstName"), "Doe")
+ * ```
+ *
+ * @param fieldName The name of a field to concatenate.
+ * @param second The second literal or expression to concatenate.
+ * @param others Additional literal or expressions to concatenate.
+ * @return A new `Expression` representing the concatenation.
+ */
+export function concat(
+  fieldName: string,
+  second: Expression | unknown,
+  ...others: Array<Expression | unknown>
+): FunctionExpression;
+
+export function concat(
+  fieldNameOrExpression: string | Expression,
+  second: Expression | unknown,
+  ...others: Array<Expression | unknown>
+): FunctionExpression {
+  return new FunctionExpression('concat', [
+    fieldOrExpression(fieldNameOrExpression),
+    valueToDefaultExpr(second),
+    ...others.map(valueToDefaultExpr)
+  ]);
+}
+
 /**
  * Creates an expression that computes the absolute value of a numeric value.
  *

packages/firestore/test/integration/api/pipeline.test.ts

@@ -138,7 +138,8 @@ import {
   sqrt,
   stringReverse,
   len as length,
-  abs
+  abs,
+  concat
 } from '../util/pipeline_export';
 
 use(chaiAsPromised);
@@ -988,6 +989,26 @@ apiDescribe.only('Pipelines', persistence => {
       });
     });
 
+    describe('concat stage', () => {
+      it('can concat fields', async () => {
+        const snapshot = await execute(
+          firestore
+            .pipeline()
+            .collection(randomCol.path)
+            .addFields(
+              concat('author', ' ', field('title')).as('display'),
+              field('author').concat(': ', field('title')).as('display2')
+            )
+            .where(equal('author', 'Douglas Adams'))
+            .select('display', 'display2')
+        );
+        expectResults(snapshot, {
+          display: "Douglas Adams The Hitchhiker's Guide to the Galaxy",
+          display2: "Douglas Adams: The Hitchhiker's Guide to the Galaxy"
+        });
+      });
+    });
+
     describe('distinct stage', () => {
       it('returns distinct values as expected', async () => {
         const snapshot = await execute(