Reordering, also known as seriation, and linear ordering, consists in assigning an order to rows/columns of a table or to vertices of a network to reveal structures. When visualizing a table as a heatmap or when visualizing a network as an adjacency matrix, finding a good order is essential to revealing high-level patterns in the data. The literature on reordering algorithms is dense but few implementations are easily available. Reorder.js is a novel JavaScript library, provided in source code with a BSD license, which provides several of these algorithms and variants to facilitate the use, adoption, and experimentation of these algorithms. Reorder.js is used in several existing Web-based applications such as Bertifier, NodeTrix, and Multipiles. It can be used with e.g. D3.js for adjacency matrices, Heatmaps, and Parallel-coordinates.